Indeed it may be ignorance from some aircraft hire operations who don't understand grass runways. Might also be experience of pilots hiring then going into grass and not taking sufficient care of the aircraft.
Grass can pose greater difficulty with low prop clearance singles and twins. Actually it is not the grass but the smoothness or otherwise of the surface and many grass strips can be quite bumpy compared to tarmac.
All the same I have flown both Seneca and Cessna310 in and out of grass strips and as they were aircraft I had some equity in did so with care and lack of problem.
A friend of mine landed at a Midlands airfield on a hard surface in a Mooney but then had to taxi on grass and suffered prop strike and an engine rebuild.
Owners have every right to apply whatever conditions they wish when hiring out but those pilots with sufficient experience should try to persuade them of their error if they are being too strict.