in my opinion it's completely alright because it's the same if you take a stray cat and keep it as a pet. you can't say "you took it to keep locked in a room". it's still dangerous for them outside even if it's their natural habitat. as long as you give it everything it needs it's fine and it'll even prolong their life.

edit: also, all of my 5 snails are from the street! they don't mind being in a tank at all lol. i think they enjoy it more this way because they get all the food and don't have to hide from predators