It's probably because Gunners are like Warriors and Rouges in that they are weapon users who use stamina as a resource instead of mana. If you give them an ability like Mana Bullet this early on, they'd be more something like a Magic Gunner or whatever, which DOES sound like an advanced class.

Also, y'know, dude gets to save up money from the ammo he doesn't need to buy. And everyone knows that money-saving magics are the most important ones. ;3