I think the idea is that "well, if someone's gonna be a dick and get our books against our expressed will, they'll probably just download them off the internet."
I actually have a Safari books online membership, and it gives me "tokens" to download books. The ironic thing is that the safari downloaded books are of poorer quality and in fewer formats than the Oreilly.com books.
Sometimes I cheat and pay $5 for the pristine and versatile oreilly book. My rationalization is that I'm entitled to the highest quality reading experience because I'm paying them $50/mo for the subscription.
Also, they spam out 50% off coupons for ebooks every week.