At this point you can consider it's intentional, but devs have never explicitly said that. This is yet again another Day 1 BUG never solved. Arcade AI isn't prone to the same ragdolling effects as human players, so any pushing, pulling, or knock down effects are largely ignored by this AI, and instead it instantly dies to these abilities.
But I didn't know this was an issue in the campaign (story mode). I believe I never used such an effect on the campaign, or if I did I really don't remember
Yeah it's kind of a bummer though, I don't play much mutliplayer, and when I do I rarely get to play heroes because I suck, so not being able to have fun and send waves of enemies flying left and right is kind of disappointing. Oh well ^^
I suggest you try either Instant Action or Coop. These are two new modes they've created that allow you to play against AI only. And these use a different AI, one that follows the rules of the game just like in a multiplayer match.
Instant Match can be accessed in the same place as Arcade, and it's just a better Arcade where you play alongside some AI against waves of AI troopers, reinforcements, and heroes even. You can customize these much like you can in Arcade (with less options though).
And Coop can be accessed from the main Play Menu, alongside Single Player and Multiplayer. In Coop you'll face the same AI as in Instant Action, but now with up to 3 teammates playing together with you (and a few AI to back you 4). I truly recommend this mode. You'll get a feeling much more akin to multiplayer matches, but without enemy human players. And you can call up friends to help you on that quest. I've played some of this mode recently, and despite only liking to play multiplayer, I found the mode to be fun and very useful. It also counts as progression for milestones and unlockables, so you can get all those levels and weapon modifications that you want without the need to fight against human players.
