Probably for balance reasons, though it could always be intended as freeLC in the future. Maybe their plan was to allow people to level up naturally, and once everyone was in a certain position of power they'd slowly roll out some crazy vehicles like the Lazer so that the game didn't start off with people buying fighter jets and annihilating newcomers right off the bat.
If anything, the tank is a lot more game breaking and dangerous. The jet is just fun.