That's irreverent. You have the choice to play a character that would best take advantage of horse combat, but you choose not to. Completely different from everyone else being given something that is off limits to you, which is what you insinuated in your example earlier.
If this was called "Warrior bonus:horse mounted combat" that would be different but as of right now, no, mages are clearly being discriminated against.
Longknife:I would assume horse combat is balanced, seeing that if your horsie gets killed, there goes a 1000 septims. Not really cheap.