PDFs are usually out a week or two after the physical book. Apparently they think the PDF being available will negatively impact hardback sales.
It only tends to if your product can't hold up to par. Many companies with confidence in their product distribute it for free in pdf form (FATE, eclipse phase, etc.) which makes buying an optional measure for folk who like what they read. So far, they seem to be doing well enough and the business model seems to be working, where the product actually is good enough to be competitive.
That said, it's ultimately a management decision and many, many economists have a paranoid fear of giving out something "for free", because they (correctly) assume there's a bunch of freeloaders out there. Then again, after seeing every single page of Siege of Vracks scanned into an anonymous forum that shall go unnamed, I figure, even with restrictions etc., freeloaders will be freeloaders, and from what I read in those threads, being able to view the product encouraged people to buy who otherwise wouldn't.