"I pay your wages!"
If it's a restaurant, clothes shop, shoe shop, or a place where communication and a bit of patter is an absolute necessity then fair enough. But in a fast food chain they're there to take your order and hand you your item and nothing more, if they seem to be struggling to that compotently, then ultimately it is just a food chain.
Anyway, somewhere like that where the staff turnover is so high, the hours are so erratic and the pay is so low, I sometimes feel the staff have a right not to interact or be friendly.
Nah that's bollocks mate.
For my sins I eat in plenty of fast food places and I don't remember anyone else not just doing the basics of being polite, saying please and thank you and showing at least a feint interest. Like I say, it's a customer facing role. I know exactly why she was behaving that why, it's because she was probably 15/20 years older than any of her colleagues including her manager and clearly didn't think her experience should see her behaving in the same way.
Thankfully, most people in fast food places, shops, newsagents etc know they need to offer at least the bare minimum to keep customers coming back. If her behaviour was seen as acceptable and her colleagues all behaved the same, genuinely you'd have fuck all people going there. Customer is exactly that, your customer. KFC isn't a necessity is it?