There are several turboprops that have "constant positive torque" - the best example is the Jetstream J41.  The short answer is that if the props don't "push back" with torque, they will windmill which will cause the wind to turn the propellers - not a good idea.  (It makes for some exciting times making descents in some aircraft though - some of them will "bottom out" at 20%.  If you wind up too high, ya gotta go around).

Brian