I personally don't need anybody behind the counter to give me a big ole fake smile and an insincere "Howdy!". I'm there to stuff my gullet, not socialize, but to each their own.

If the employees are really making fun, well, they shouldn't be obviously.
But you can't figure out why the manager allows it continue?
Million dollar question: have YOU spoken to the manager? If not, there's your answer. Because he or she may not know about it.
Speak up. Problem solved.
But do it on the sly because if the employees are as 'mean' as you say and they even for one second think you ratted them out, I'd find another location to patronize.
You may get something special in your next sub and I don't mean extra pickles.