The problem is that Cycles isn't implemented equally in all three forms. If you want the simplest solution with the most features for the greatest number of users, you go with the CPU.

Another thing to bear in mind is which features are available. In the most recent released version of Cycles, subsurface scattering is only implemented in CPU, experimental on NVIDIA cards (and may not work) and unavailable for ATI cards. Poser still, primarily, deals with figures. That have skin. Which has subsurface scattering. Having a "realistic" rendering engine, which Cycles is supposed to be for, that can't do realistic skin is kind of counterproductive.