I use it. They're not reserved or exclusive, just there for accessibility. In fact, sometimes I'm guilty of just using it without thinking even if there are other regular stalls available. But it really doesn't make sense to have a big line of people and ignore the accessible stall. I know that sometimes disabilities are invisible, but unless it is an emergency and they are willing to speak up, no one can be faulted for not offering it up to someone with an invisible disability. If I were in the same line with a person behind me who I visibly thought might need to use it, and the other stalls were being used and the accessible stall was open, I would offer them to go ahead of me. Unless I had an emergency, in which case I would probably say so apologetically. It's not required, but it's polite and courteous - I could use any of the soon-to-be opened stalls but presumably she could not. I feel like it would be rude and thoughtless to take it while other stalls opened up that she couldn't use while she waited for me to finish. Life is difficult enough with a disability; if I can do something to make it a little easier for someone, why not?
The woman in your story was rude. If I'd walked out and seen her, I'd have politely apologized and stated the other stalls were filled when I came in. The polite thing for her to do would have been to say, "No problem" and take her turn without going on a tirade. Rude people come in all varieties. I understand it was probably frustrating for her, but sometimes we all just have to deal with being frustrated without flipping out.