First off I don't do this sort of thing. I have enough trouble trying to line up myself. However....
Someone show me where it states in the rule book that this is wrong. A bowler can throw a ball in practice anywhere they choose. If he is a right hander and chooses to throw on the left side, thats within the rules. As long as the ball is legal.
Is this okay when its a right hander versus right hander? It has been stated here in a post some time ago where a professional admitted to doing this in match play. His opponent was striking and he wasn't. What did he do? He intentionally changed balls and screwed up his opponents line and proceeded to win the match.
I agree that this not a good tactic but until the rules change it is now apart of the game and something bowlers have contend with.