Why are shard chats allowed?

They will never do anything about it.  Your only hope is if you can manage to start sniping people at their PO's and make yourself a big enough nuisance until they offer for you to join.

Like everything, they vary.  The fleet shard chat I am in is relatively friendly we've invited a lot of people in the shard over time.  Not all of them have accepted the invitation.  I don't think we've ever turned anyone down who asked to join us.  

People in the chat who have the same payout time are on a rotation for who gets to be the highest spot.  It works relatively smoothly.

They are not allowed not not allowed. Developers cannot stop it. Players are coordinating outside the game. 

Yeah, this. 

Tomsteros has some bad luck with that shard. Those are some terrible attitudes, there & it's not like shard-switching is possible. 

Best advice is

1. Learn to live with it if you can, or
2. change your PO to some obscure time and keep changing until you find a PO that they don't care about. If any. It might not work, but it might. 

I wouldn’t say they are “allowed” more there is very little CG can do as communication and organisation is held outside the game.

I would say that’s petty from them and they will tire eventually. The key aim of shards  is to benefit players by improving efficiency of rank 1 to receive 400 crystals. Extra battles cost crystals so keep climbing without using crystals incase you are sniped and eventually they will not want to waste the extra battles and crystals. If we are talking from 20ish they will need approx 5 battles minimum in their climb. So any attacks on you outside their climb will require refreshing and spending crystals.

Double check they are not sniping back because you dropped them during their climb. If your climb is close to when they are actively climbing experiment with another time because it’s not worth head butting. If someone drops me as I am climbing I will go through them again, I usually clock when their PO is and quickest way to upset me is to climb outside of PO and snipe me in mine. I have only sniped others who persisted to snipe me after I reached out.

I've commented on this before, but making the rewards for Fleet Arena the same as those for Character Arena would solve the issue. In turn, either increase the rewards for GAC or add a Fleet-exclusive Grand Arena.