Chicken6 on a C150/2 and C172 the tailplane has 2 spars, the rear one is the main spar and carries the elevator hinges. If you push down on the tailplane, you are exerting leverage sufficient to lift the nose, ie equivalent to a sizeable proportion of the weight of the aircraft. This force is being exerted mainly onto the secondary spar on one side of the tailplane. This can exceed the loads that the spar and it's mounting on the fuselage was designed to take. End result, expensive and potentially dangerous cracks in the spar and or mountings. A secondary effect is the twisting moment applied to the fuselage which was never designed to accommodate rotary forces.
These cracks do happen, I've seen a stripped down C150 tailplane and it wasn't pretty. Fortunately it wasn't mine!
It is far better to push down on the fuselage than the tailplane because the load is being taken by a structure that is designed to take vertical forces.