Okay I went back to Sonic and had a small talk with an assistant manager. She verified that tips are welcome as a nice gesture but employees are certainly not to ask for them. Tips are also given at the drive-thru as well. As for the refills? Yes, Sonic does give free refills.
My thoughts on Sonic after all this drama:
I'd rather eat better food for the same price at our local Mexican restaurants who also have free refills and work just as hard or harder for their tips. No time to sit? I'll go to any other drive-thru.