A couple points worth mentioning:
1. For most retail clerks, the choice to leave is often "stay in a shitty job I hate" vs. "become homeless and starve (and potentially my family as well) because there is a lack of other jobs, high competition for them, and I live paycheck-to-paycheck because the jobs I can get pay minimum wage with no benefits."
2. It might be different overseas, but in the US at least the vast majority of retail clerks are paid hourly at the minimum wage. In fact, many retail employers do their best to have managers play games with people's scheduled hours to keep them under the amount that triggers benefits and overtime.
So in many (dare I say most?) cases, yes, it is too much to expect some nice disposition by default. That said, I would hope most people, when treated nicely by customers with realistic expectations, would respond in-kind.
I'm not sure why talking about emotional labor, smiling and singing, suddenly brought on doubts about decency, niceness, and assholeness in the following reply. I see that as very stressful expectations to have for somebody.
Smiling is a communication of internal feelings. It puts pressure on me as the customer as well when I know my mere presence is an on-button for an emotional act for people. How draining for both parties.
Yes, I get it - working as a clerk in a store like Walmart is not the most luxurious job in the world. But I, as a client, is not at fault that the clerk is not a billionaire hedge fund manager (neither am I btw). On the contrary, the money that I bring in are partially paying his - admittedly, far from luxurious, but still nonzero - salary. He is not forced to work this job, he could leave at any moment (he'd have to give up the salary of course). So is it really too much to expect some nice disposition towards customers which pay your living? I really don't understand how it's unreasonable and "degrading" to expect that. Singing maybe going too far (OTOH, the video looks like a dreaded "teambuilding exercise", that's the whole other story), but a nice smile?