It is mostly a control issue I think. Making someone stand for long periods demonstrates a greater amount of control over that person. It is a constand reminder of their place in the organization.
When I owned my own shop the cashiers had stools to sit on and nothing negative ever came about because of it. The were not less efficient. This only works if you have someone else to bag groceries however. If the cashier also has to put the items in a sack then they have to stand in order to do this job. You can't reach far enough to do this while seated.
A surprising number of customers commenting on this practice however. They usually told cashiers how lucky they were to get to sit all day instead of working hard. It's really hard work not throttling someone after a comment like that so they earned their pay on that alone.
John Thornton