OP I think you had every right to be annoyed by that. I worked in a small store (comic book shop) in a strip mall, there were 2 employees in the whole place, myself and my manager. I worked alone on her days off, she worked alone on mine. It was up to us to make sure we took our breaks & lunches and got to use the bathroom. Sometimes it was crazy in there and I would actually have to tell people in the store "I'm sorry, I have to use the bathroom, can you please wait outside for 5 minutes?" Most times it wasn't a problem, only once did a guy get irate with me. I apologized and let him know it would only be a few minutes, he wanted me to lock him in with me, I couldn't do that and had to ask him again to leave. He finally huffed off after I explained again why I needed him to wait outside. I was done in under 2 minutes but he just left. Everyone else understood and was fine waiting. As for my breaks/lunch I never locked the door when I took a lunch, I just ate behind the counter and if anyone came in I helped them, I did the same for breaks. Yes, there were days I didn't get to take any breaks or lunches, but you know what? I dealt with it. I knew when I accepted the job that I would have days like that and I dealth with it. I made jokes with my customers while eating a sandwich or whatever, maybe I was lucky and my customers were just nicer people but really, it doesn't take much to be polite and say "I have to take a break, give me 15 minutes". The person working the salon only needed to say she needed to grab a quick bite to eat, to say she needed to use the bathroom, to say she needed to take a break by law, whatever, she should have said something and not left everyone hanging. It's common courtesy, and yes I would have been as annoyed as the OP, and yes, we go to a Great Clips type of salon for all of our haircuts. I understand she may have felt overwhelmed and not wanted to "deal with" people getting annoyed but there are so many ways to counter that, she could simply have told people coming in "I'm really sorry, I'm short handed because xxxx and I'll need to take a break after this client, if you'd like to wait please sign in, if not I completely understand", but she didn't, it sounds like she just ignored everyone and walked away. That is what is not OK in this instance, that she simply ignored everyone who was waiting.
As for the OP having a sense of self entitlement because she's unhappy with the service she got at a salon, I think that's ridiculous! She is upset because she got ignored, and she has every right to be upset by that. She also has every reason to not want to cross 8 lanes of traffic going 50 mph to get to another salon, jeez, I wouldn't want to cross that either!!