The situation varies from state to state. In Florida, many businesses are required to have public restrooms. So you should be able to do your business without being coerced into buying that $10 drink.
There is no state law that forces restaurants to allow people to use those restrooms.
Restaurants can refuse access to non-customers.
And many do, because of the repeated experiences of non-customers destroying the restrooms. They aren't doing it just to be dicks to people who need to pee but don't want to buy something.
This is why we can't have nice things.
there are plenty of roboloos that clean themselves now though, so that's not really an excuse anymore.
https://www.flrules.org/gateway/readFile.asp?sid=0&tid=27562...
You read it and might think guests applies to non paying customers. I read it and see it only applying to their guests, i.e. patrons of their restaurant. 3rd person might interpret as simply meaning the restaurant determines who is a guest and it can swing either way. 4th person is a lawyer and thinks some random other part of this document or other law or court case defines guest therefore his interpretation is correct. 5th person is like, we don't care, public access to toilets is a human right and proclaims the first 4 are bigots and such restaurants are to be sued. 6th person starts an organization to catalog all misbehaving restaurants and labels them discrimatory. And so it goes, and will go on till the end of time.
nicest places? good signage, no need to even interact with a human, restrooms that look like the Taj Mahal.
middling places? restrooms exist, but you might have to ask
bad places? restrooms are hidden, maybe if the clerk is nice you can use theirs, or you have to buy something
horrible places? restrooms that do exist look like something from a horror movie, they're locked stronger than Fort Knox, and even then you have to buy, beg just to borrow the key