The tip topic always seems to beat to death. In most cases, there is certainly no obligation ever to tip except, in the most common instance, where it is a standard to add for large group checks.
You're intelligent enough to realize that it is more likely than not a low paying job at Sonic. Social and cultural mores aside, if you have the wherewithal it's alright to help a guy out that is trying to earn a living. Really - it's okay. If you choose not to, that alright as well.
It's okay to help somebody out just because you can.