You'd have to be an idiot to have it automatically sharing files you downloaded. It's not just because that could result in unwittingly sharing spam, but because it could result in unwittingly sharing illegal stuff. Nevermind bootleg mp3s; there are child porn pictures floating around out there, not all of them clearly labeled. It's a very bad idea not to vet all files before making them shared. Even so you might get in trouble for possession of questionable files you got by accident; the penalties for distribution, however, tend to be far worse than those for mere possession.