Or could it be that they're bored and unmotivated? Most of the "unsmiling" employees I've seen at the less-than-desirable McD's locations and other not-great-run fast food places seemed to have a bored, aloof, and/or "out of it" look, not angry or sad look.
Just as there are "bad" managers out there, whether you choose to admit it or not, there are "bad" employees too. But not all places are shitty-run. I've also been to places where the managers and employees do show an interest in keeping their store clean, maintaining a quality standard in food service, and warmly receiving customers, ...and giving them the service they have a RIGHT to expect.
I've read posts by caring employees and managers in these rooms, more power to them for their commitment and support. As for the negative posts, you can get an idea of which ones are having real problems with their employers, and which just plain have bad attitudes/work ethics. In one post last year, one guy criticized all other employees in his store for "taking their jobs seriously", he also called them "stupid". In the Workers' room, one employee confessed to smoking pot on the job, and a recent contributor advised a customer to "go somewhere else" when he was asked what he recommended [on the menu].
And in reply to Drive Thru Girl, it's not impossible to find a good fast food place as your reply to one of my posts makes out. In fact, I've been to plenty of them. Again, I've seen the best and the worst of employees and managers.
To close, I can understand it's difficult to smile and be in good spirits when you work for nasty or indifferent management. But that isn't always the reason.