Nothing to do with whether the booze is included or cash, this has to do with cutting someone off.

Nothing to do with limiting passengers to one drink on the first pass when there are limited supplies, thus allowing others a chance too.

This is solely about FA's complying with both their legal and employer rules requirement about not serving people who "appear impaired." The key is the word "appear."

The same people who are offended when shut off would also be the first people to sue if they stumble and are injured after offloading.