I believe the true intent of this drop in gameshape is another deterrent for day traders but the 'realness' factor of a drop in game shape due to new surroundings is plausible but in that case it should also be plausible to have an increase in gameshape due to new surroundings e.g. James harden to houston (more minutes, more opportunity, change from 6th man to key player).
I agree there should be no penalty in gameshape if you buy your own player but I feel the argument against it would be to be more careful on your pricing of sale players and avoid bidding on your players.