The difference between these two is there’s an implied probability of guilt, which is a dangerous view because it allows you to treat people who haven’t been directly proven guilty worse on the basis that you’re mistreating a population more likely to contain guilty people. The presumption of innocence isn’t objective, it’s an important psychological tactic to help avoid such behavior. That’s why we should use that language.

Edit: in practice the legal system doesn’t behave this way, but I’m still wary of using different terminology because it seems like it concedes ground.

But it doesn't matter. That changes nothing more than semantics, which doesn't explain or justify anything.

Oh, I agree with you. I'm just stating how the legal system works. Doesn't make it right though.

Well, no, you're not actually stating how the legal system works, you're just stating which of two words it uses to mean the exact same thing -- it works the same way regardless.