Of course, they put your name & other info on every page of the PDF generated for you. That probably works as a general "shaming" mechanism, such that most folks won't share indiscriminately.
If I put my copy of an O'Reilly ebook online for anyone to grab, it's obvious the copy floating around is mine and I'm a jerk. But I can still give a copy of the PDF to a few friends and encourage them to buy a copy if they like it.
And, there's absolutely no way this is going to somehow bone me in the future. The authentication server isn't going to go down, it's not going to mistakenly think I lent out my "last copy" or lock me out of my purchase because I don't have internet access.
In other words, you better not share, unless you've got a lot of money laying around...
Pragmatic Bookshelf, Manning and Packt (out of the e-books I have handy for quick checking) do put your name/address on the bottom of every page to discourage piracy.
Edit: Specifics for each publisher, for anyone who's curious:
Manning - Name and email address on each page
Pragmatic Bookshelf - Name on each page
Packt - Name, physical address and purchase date on each pageThe other (regular, relased) books from ORA don't.
I certainly do share music with my girlfriend, which presumably is considered piracy by the RIAA, etc., along with ripping the CDs I bought before DRM-free music was widely available.
The "Safari Books Online" is another services with many DRM-like techniques but this was a previous services where O'Reilley Media is member along with other book editors. I used it in the past but that was a pain to use it.
I suppose O'Reilly now focus on the first service only as this is the one generating revenue without DRMs or watermarking techniques or additional limitations. I buy much more from their non-DRM offer (it's convenient, low-cost and many formats are available).