I rarely find myself at sonic anymore, but I understand where you are coming from. Sonic Corporation chooses to pay their servers less than that of McDonald's employees. To me, its an ingenious way for the Sonic Corp to cut down on their expenses.
If your employees cost that much less, but they will still work because the customer base will support them enough to make it work then they are golden. If everyone, unanimously stopped tipping sonic car-hops Sonic couldn't employee their people at that wage level.
I bet Kyle's brother, on average, makes more than the McDonalds employee across the street.
Just their business model. Clever until too many people like yourself come along.