I'm not 100% sure where all leaks come from, but I'm assuming a lot are journalists/industry people who decide to upload their promo copy?
I've got in front of me a promo copy of the new Boys Noize album, and there's an aural watermark type thing, saying 'this is a promo copy' across various different EQ levels and frequencies, something that would be pretty hard to remove. Is it not possible for record labels to put this on every promotional album they send out? Its still possible to hear and review the record, but you certainly wouldn't want to keep that version. Or is that just not viable?