runematic:
You're a lot more enlightened than a lot of business owners! Many either do not "know how it is" or they just figure that "how it is" is good enough for "those people." The world needs more people like you. I'm sorry that this woman made a choice she didn't have to make, and that it affected you negatively. If it helps, you could think of it as a case of someone who has been mistreated by others so long that it didn't even occur to her that she might ask you for the use of your facilities, and get treated well.
Incidentally, my own pet peeve is people in upscale surroundings who treat a restroom as if it was in a filthy gas station - which is then the kind of restroom it turns into. I once worked for a company at One Buckhead Plaza, a very upscale Atlanta building. Restrooms were immaculate in the morning, and serviced a couple of times a day. But guys who refused to flush (because the handle might have germs, and because they damn well weren't going to wash their hands or anything sensible like that), or wouldn't stand close to the urinals (because the last guy didn't flush it) or who threw paper towels on the floor (didn't want to touch the trash can lid), turned it into a pigsty within an hour or so.
I have much more sympathy for homeless people who do what they gotta do in the absence of facilities than I do for so-called "nice" people who refuse to use a clean facility properly. Stand close, use, flush, wash hands, throw paper towel away in the trash can - what's so difficult about that?