You can just say, “If you’re not with us, you’re against us.” It’s shorter, and less weasely.
Out of curiosity, who do you think "us" is in this case?
You're basically advocating that someone should be on one pole or the other. I think your premise seems dishonest:
First you gave an assumption about an 'unjust' system and explain why not choosing a side is bad, but then later on you say that not choosing a side isn't 'inherently wrong' to try to not seem judgemental. It comes across a little manipulative tbh.
And for people outside the system, the "us" is often both the left and right interchangeably.