Like you do not sign a legally binding document that puts severe restrictions on you and people like you without reading it fully and getting everything in detail because “oh if there’s something I don’t like I can change it later”?? You are signing as-is you are bound to what that document says right now and even if its possible to change it that doesn’t mean you will be able to and it means you are consenting to everything in it not just to good parts there are now. Like if Steve signs it that means he’s consenting to Wanda being put under house arrest (and if she breaks that something worse) something he doesn’t believe in because it reminds him of internment and he can’t be like “well I didn’t mean that part I only agreed to the parts I liked we have to change that” because no he agreed to it! He’s giving them the power to do that. Like as it turns out even Tony didn’t know everything in it because he had no idea that they would lock everyone up in the Raft aka Supervillian Alcatraz and even though he says he doesn’t agree with it he apparently can’t (or won’t) change it. That is why signing a document you have not fully understood or agree with is a Bad Idea. Steve wasn’t totally pure in his motivations but like signing it and “changing it later” if you don’t like it is such a terrible alternative.