I don't see how this is an exploit...
Warlocks and Pallies have quests that give them freem 150 riding skill + a mount.
Blizzard SPECIFICALLY lowered the prices on mounts so that warlock and paladins, along with everyone else, could have multiple mounts at a reasonable price.
I'm pretty sure using something Blizzard implimented, exectly how they intended it you be used, isn't exploiting.