Let's say I generate a new address and by some miracle it has Bitcoin already in it belonging to somebody else. Is it legal to just take it? I understand those odds are very low, no need to lecture me, I'm just interested.
Stealing Bitcoin is illegal but the truth is that nobody can do something about it and that is maybe why some people think that you can just do it.
It is just as illegal as stealing something else only the police have no influence on it.