Pentax K mount seems like a slightly odd choice, but that may just be a North American bias where the only real Pentax sales in the last 20 years have been K1000's and clones to first-time photography students. I wonder if Pentax has other more sophisticated K-mount cameras they sell in Japan only?
Probably a low marginal cost in offering K-mount as well as M42. And don't Pentax digi SLRs accept K-mount? I couldn't believe how good the 25/2.8 made the Nikon D70 look.