So if you can only learn to fly on a trainer then you have buy another plan after you learn to fly. Needing another plane means there is a very good chance you will go to his store to purchase it which means he gets to sell 2 planes instead of one.
Chances are if you don't start on a trainer you are going to be buying far more than 2 planes to make up for the crashes... I would say the probably the majority of clubs have a similiar rule, if not written at least understood. training on a P-51 is NOT a good idea!
besides, with tower hobbies and the host of other great internet stores, it isn't much of a concern. and trainers are still useful after you move on to other planes, can't ever have too many!
*R.I.P. avistar, recently departed, wings took on a dihedral angle of ~60 degrees while in flight...*