If you read up on it, the ATF has a terrible reputation for trying to enforce rules that aren't on the books, making laws regarding alcohol, tobacco & firearms (only Congress is supposed to make laws,) and being used as the Clinton's personal army during the 90's (they were supposed to be enforcing tax law, not gunning down people.)
But that's IRL. In the game: I don't know what the atf is and join it for the free booze. It's not a good game...