You'll run into that one if you try to go the other way as well - RW to FW.
It's probably because of the prejudices held by those in powerful positions that do not have the multi role capability themselves. It could also be the ruling of insurance companies poorly advised by such people.
Like you, I fail to see why there is no recognition. There might be some differences in the flight characteristics between piston and turbine powered machines but these should be addressed at type endorsement level. If you have flown a turbine anything you are further down the road to understanding the principles, and the different manner in which they deliver their power, than someone who has not. Even the transition from piston to turbine is held to be some sort of magical rite of passage requiring superior ability.
It's all a load of bollocks!