No, it's not for after conviction. It's for when I'm being held before conviction. My lawyer gets to make the government prove that they have a reason to hold me. It's why a cop can't throw me in prison for six months for looking at him funny.
It may have some application after conviction too. I'm not a lawyer. This sort of thing is taught in high school, though, and it's important enough that it should be remembered.
I think you (and everyone else) have lost the thread, mate. He was replying directly to the question "why can't they just stay in Europe." The answer is "Europe had immigration laws."
I don't see anything wrong with this exchange.