The software itself is legal, it's just the content being shared that is potentially illegal. Anybody who distributes anything without permission from the copyright owner is acting illegally. For example, you upload your band's (if you had one) song for anyone to download and distribute freely - that is legal. You, as the copyright owner are allowing it. You could also rescind that permission or restrict it. If you upload your collection of Britney Spears that is illegal, since you are not the copyright owner. These sites absolve themselves from responsibility for any illegal downloading taking place by posting a notice regarding copyrighted material somewhere on the site.