There's at the very least a lot of social and managerial pressure to act cheerful. Even if you didn't do it, many people would because it's effectively required of them. Something being technically a choice doesn't remove the social pressures that make many feel that it's not a choice. Plus I wouldn't be surprised if it's been used as rationale to fire people before, which means that for some, it isn't a choice (and most people are scared of losing their jobs, so won't try to find out).

In tipped roles, it would definitely result in less pay.