You are assuming that all economic activity is rational. One only needs to look at Bitcoin price to know otherwise.
Employer isn't actually getting any value out of making cashiers stand, and in most other countries, it is a sitting job. So far as I can tell, the only reason why US does it is because "we have always done it this way", which is also the ultimate rationale for a lot of other similar bullshit.
The workers, realistically, don't have any choice if every employer does this. They can take the work as it is, or they can starve, especially seeing how US doesn't have full-fledged welfare.
