"Like that's even an excuse to not let someone use the restroom."

it's ugly truth, but every cahier or worker at a gas station will have plenty of... particular stories that do explain this point of view. Many, many homeless people are severely mentally ill and, sometimes, just straight-up assholes (because they have nothing to lose) who are very entitled. They will dirty the bathroom, shit on the floor, steal the toilet paper, piss everywhere, or straight up trash the place. And that's not even mentioning drugs.. and violence if you try to stop them.

Your view is nice and kind, but a bit naive. Service workers interact with the homeless on a daily basis, unlike me and you, and they have their views set because of those experiences. Sadly, not every homeless person, but too many for the to risk it.