Pentax/Hoya would be very stupid if they let Tokina compete with them with a budget version of their own patent. And before anyone repeat the stupid claim that Pentax did not design this lens, here is the patent:
esp@cenet — Original document
You can compare from Diminitrovs pages that it is indeed the DA*50-135/2.8 that is described in that patent:
http://www.bdimitrov.de/kmp/lenses/zooms/_optics/50-135f2.8.gif
Pentax name on the Patent, not Tokina. Probably Tokina payed Pentax for the permission to use the lens design, but they do it with their own coating, and without weather sealing and SDM, and of course, in that contract Pentax must have specified that Tokina were not allowed to build the lens with K mount. Don't know if Tokina payed once for the design or if Pentax gets a share of each lens Tokina sell, or both. Hope they sold it expensively. It's a nice lens.
As far as I understand, they have the same agreement for the DA*16-50, DA10-17 fisheye and DFA 100 macro, which Pentax designed. The oposit applies for the DA12-24, which Tokina designed, but Pentax builds its own version.
Another sort of colaboration is when Pentax let Tamron build the 18-250 but put the Pentax logo on it.
This sort of colaboration is as old as lens production. Many lens solutions have completely legally appeared with many different labels. Pentax had for example a verions of the
plastic fantastic 100mm f3.5 macro lens that appears in numerous names including Soligor, Phoenix etc. Severall of Ricohs K mount lenses were rebadged Pentax lenses. Vivitar never built any lenses of their own. And before Pentax even built own cameras they built lenses for Konica and Nikon (or their ancestor companies).
You can compare with the car industry were cars of different models and labels use the same engines or frame (not sure if it is the right English name). As a consumer you might not like it, but it is cost efficient, and you would in the end pay more if they did not share costs for development.
You know, if you absolutely want the Tokina version, you can buy it in Nikon mount and try if it fits. Some Nikon lenses do, mostly older ones, but also some new. But I wouldn't recommend it