¶ 3-304.1

https://www.fda.gov/media/110822/download

tl;dr If the used cup won't touch the pouring utensil, it's OK to reuse; otherwise, you get a new cup.

In retrospect, this is what I observe as a customer in most cases.

i don’t know except that’s is USA-wide. it came up in berkeley recently when the city required restaurants to charge extra for disposable containers. the issue of customers bringing their own had to be dealt with.

ever notice in a buffet you are not allowed to reuse your previous dish?

Weirdly, the two buffets I’ve been to don’t make this rule apparent and only until I was chastised by the cooking staff did i know. I hate having to learn these hidden rules the hard way.