here's the thing:
There's no reason to limit the ability to contribute from the
players' perspectives, but I do see a reason for the limit from a
game designer's perspective.
Think about what would happen if a newbie was enrolled in a guild that had just begun to complete Heroic Rancor. AAT is taking forever, but it gets done. You sign up, you do your (tiny) part in the raids, and you get a reward for completion of Heroic Rancor even though your participation was unnecessary and effectively insignificant.
Well, okay. You get rewards for something to which you didn't really contribute, but the rewards are ranked, and if you contribute very little compared to most guild members, you'll be ranked 30th or worse.
Now, ranking 30 or 40 or even 50 in a rancor raid is going to get you fairly little, but it gets you something. And if you've just become guild-eligible, you're level 28 and the rewards you're getting are pretty great stuff. You can collect them and over time you can have lots of gear waiting before you get your very first toon to g9 which is when that gear starts to become important (usually you only need one raid-limited piece at g8, sometimes none).
How do they stop that? Well, they don't, but they do put a 7* toon requirement on the Heroic Raid. If people are finishing off a high-reward raid for you, you've got to be more developed than you are at level 28. Social pressure also keeps non-contributing members out of guilds that are completing Heroic Rancor from taking on the most junior members.
But what about TB? In TB you all get the same rewards. If you are TB eligible at level 28, the rewards are even greater than Heroic Rancor (at least, if you're in a guild that can complete Heroic Rancor). You drop in, deploy forces in TB-phase1, earn 50k territory points, and then a few days later you collect the same reward as the players who have been at the game for more than a year and have 10-20 toons waiting on gear12.
Now, not allowing you to collect the same rewards would completely undermine the whole point of territory battles: group effort, equal rewards. But allowing level 28 characters to quickly gain access to a whole host of gear that most of us have been grinding for months will make that player disproportionately powerful at level 75 compared with where guild mates were when they were level 75.
And that affects a huge number of other things as well - many events that are tailored for characters "level 75 and up" are effectively impossible for any player, even with an ideal play-strategy, at level 75. You can spend big to defeat that content at level 75, but otherwise you need better mods, better gear, etc. etc.
And you can't get to g12 without being level 84 or 85 b/c some gear at tier 11 requires those levels to equip. But you can have g12 waiting for you, so that the moment you become level 84/85 you can equip that stuff and become a juggernaut without waiting like others did.
Now, if you start a new account today, it will always be behind an account that is level 65 today ...
...but if it got TB rewards, it would always be ahead of the power curve for each bit of new content that unlocks at a specific level.
I'm not really sure how to fix this if we're to allow all players to participate.
One suggestion:
Game Devs could put a minimum contribution on accessing reward ranks. If you one-star a single territory, your guild earns Rank 1 rewards, which are very meager, but not nothing. The more stars you collect (up to 3 per territory, with 15 territories over the course of the 6 phases = 3 x 15 = 45 max) the better your guild rewards. Everyone collects the same rewards, as long as they met a certain minimum contribution.
To earn rank 1 rewards, there is no minimum. If you guild got one star for the whole TB, you get that reward even if you contributed only a single Territory Point (which is impossible b/c before fighting, and presumably losing, you still have to deploy a single toon and get that toon's power added to TPs...and no toon is worth only a single point).
To earn rank 15 rewards, the minimum is set at something easy to accomplish for level 70 or level 75 players.
There are no new minimums above rank 15 - if you contribute at a level that is easy for a level 75 player to accomplish, you get the guild rewards, even if that's a maxed out Rank 45 reward.
This way, there is no competition. You can't knock one player out of rewards by taking their rank from them, but you don't qualify for fully equal rewards unless you're playing really, really hard at level 65 and doing everything you can for your guild ... or playing very little and are level 85.
It's hard to say whether this would better preserve the spirit of "equal rewards". I think it might. I like it because arbitrary on/off switches like "achieve level 65" don't value how a player contributes to their guild. The level 65 player contributing their best in TB is also going to be contributing lots of raid tickets, and is likely a player that those with more developed rosters and higher levels are going to want to keep around even if they can't do 1% on HAAT (or Rancor!). The level 65 player making an average effort still can't access rank 45 rewards, and at level 60 it's probably impossible. But you can score some TPs and meet the minimum for a higher reward than someone level 28 who just joined a guild.
But just because this is, in some senses, a "better" solution (because it prioritizes and incentivizes your efforts as much as passive contributions like merely deploying a high-level toon) doesn't mean it's automatically the best compromise in this case. Remember that we don't want to reward effort too much. Equal rewards, right? There's a lot of good in there, a way to eliminate competition and hard feelings (or at least shove them off to the Raid side of the guild) and encourage cooperation and even friendship. Anything less than equal isn't really equal. So whether it's a hard requirement for mere participation or a graduated requirement up to the point where you an meet a minimum contribution, either way the Devs have decided that the game content won't be balanced for newer players if special gear and special toon shards can be collected at level 28. If level 65 is where the best turning point is for an on/off switch to prevent too much early accumulation of late-stage gear, then phasing in accumulation of that gear still has to average out under a graduated system to what would be earned in a nothing at level 64/everything at level 65 system.
It's really not that much harder to code my system, but since game balance for existing content is going to be considered, and on average things are going to be the same under either system, it's very likely that the Devs will leave the system as is. And I can't blame them.
