Businesses have policies that you're supposed to do this for the same reason that businesses have policies that you're not supposed to spit on people; basic human decency needs to be encoded because some minority of people won't go for it otherwise.
Being pleasant to strangers even when you're having a bad day is basic human decency, and of course it's supposed to go both ways. If I'm a customer at a burger shop and I'm having a bad day, I still smile and say thank you when a stranger hands me a burger or holds a door for me in public, not because any boss is forcing me to do it but because this is simply the decent thing to do. Just because I'm having a bad day doesn't mean that I should make that somebody else's problem. Spreading misery around doesn't help me one iota, and only makes things worse for everybody. Having a surly attitude with strangers is basically social vandalism.
This is true outside of commercial contexts, and still true within commercial contexts. You're complaining about the power dynamics of the commercial context, which fine, fair enough, but the basic principle of treating people decently instead of making your problems into their problems applies in all public contexts, including the customer side of those commercial interactions and also when no money is being exchanged at all. And of course some minority of the public won't reciprocate, there are some people who treat service workers like trash and unload their problems on those workers... they're wrong for doing it. Those kind of people existing are the reason businesses have to encode common decency into their rules and policies.
Edit for Stefan's remark: "It's the least the servant class can do - fake joyfulness to avoid saddening their betters, right?"
"Servant class"? What I said is that everybody should be decent to other people. Read my comment again if you didn't catch this the first time. It's supposed to go both ways. It isn't a matter of class at all.