While I am not a bog fan of the excessive tipping society the US has become, it does exist and I'm because of that I'm not keen on the attitude that you don't tip the car hop because they get paid to bring food.
In essence, that's exactly what the pizza delivery guy does too and yet a tip is expected (despite the fact that the pizza place also charges a delivery fee). The cab driver is paid to drive you to your destination, the hair dresser paid to do your hair, etc....but we tip.
Heck gamblers even deal poker dealers after a winning hand...but their job is to deal cards. And that's all they freakin did.
I do not tip those places that add tip jars. For the most part I tend to avoid them. There is one exception and that's a local BBQ joint that a husband and wife team run and are the only employees. I like them, and like local businesses so I tip.
As for carhops, I too tip when I have cash to do so (and the hop isn't in a big hurry to hand me my meal and leave).
Yes the minimum wage for waitstaff and service industry employees can be below the normal minimum wage, but many states require that employers still ensure that those employees still earn minimum wage once their tips are counted.