Sounds to me like this is just a bald assertion with zero evidence. (On a web forum no less? I'm shocked. Truly.
EDIT: Not just a bald assertion, a patently false one as well. It took me about 60 seconds to find a torrent of a 3gb set of O'Reilly books. Somehow I doubt that downloading it would qualify as a legal act.
We don't know O'Reilly's numbers, we don't know if sales and/or revenues and/or profits would go up or down if they added DRM.
In addition, what works for O'Reilly may not work for others. They're happy to write off unauthorized downloads as a type of promotional cost. Meanwhile, O'Reilly pays a low royalty rate (10%) aren't reimbursing authors for possible "lost sales," and has a tech-savvy and sensitive audience. All of these are factors largely unique to them and their business model.
In contrast, a publisher like Penguin, with a large multinational business and dozens of imprints, and authors with more substantial royalties who will get much more irked by lost sales, may just as easily have bad results as good without DRM.
The only thing O'Reilly shows is that for their
business, they are willing to take the hit. Perhaps this extends to other publishers, perhaps not, their experience isn't really proof either way.