In the US it’s pretty rare to sit in any retail or restaurant job while you’re working with customers. If you want to sit down, usually you have to get someone to cover your post so you can take a break.
The idea is that if you’re not helping customers, you should be doing something else that’s useful. Like in a coffeehouse, cleaning/restocking the tables and behind the bar, or in retail, straightening the shelves. If the company can’t find something for all the employees to do, it means business is too slow to justify having so many people on the clock, and they’ll often send someone home early if that happens.
I think @JLeslie is also right about the perception of laziness. Businesses want it to look like the staff is ready to help with anything at a moment’s notice. I don’t think sitting down every so often stops anyone from being helpful, but that’s the standard.
At more than one of my past jobs, the company would send secret shoppers around to make sure everyone was on task. One of the items was, “is anyone sitting down?”