Excuse me; the people I KNOW understand that using the bit torrents (or whatever) to get ebooks is illegal. Just like they knew the old Napster was illegal. They just don't care. But then again, the people I know in this age group who read tend to have at least a minimal sense of situational awareness.
It is true you don't know whether a copy is legal if you don't know how it was obtained; generally one takes on trust that a reputable company like Amazon has obtained things legally.