Well Zorz, either you don't try to find it hard enough or smart enough.. I can find pretty much a lot of ebook by O'Reilly in the darknet circulation. Heck, I even find that one guy here at mobileread linking his epub format O'Reilly book to his personal website. Maybe the guy didn't have intention to share it illegally, and i can't remember what he wants to do with thread he started (I think it's related with epub formatting and his ereader device).
Beside that, yes, pdf is the most common format that will show up when you search, but epub and some chm format is also out there in the darknet.
And according to your thread title which suggest that drm is useless because O'Reilly books never appear in ebook piracy circles, I don't think that proves it at all. I think that (and like many people here who have same opinion) when ebook is reasonably priced, have good quality and formatting, no stupid geographic restriction, and convenienced to purchased, people will eventually buy it in legit way, rather than have to go through the hoops of finding pirated ebook. That way, drm will be useless because it just give an unnecessary inconvenience