It falls under the "this is why we can't have nice things" corollary of human behavior.

If you've never worked in a public setting (esp food service) you might have never been the person who had to go clean the bathroom. And sometimes you get asked to go clean that bathroom after an inexplicable incident and you find yourself staring into the abyss, wondering how there can be both so much feces and how it got to all those places.

Everyone assumes it's because establishments are being 'anti-homeless' or some other exclusionary behavior. No. It's because some person or persons ruined it for everyone else. They could have just gone in, used the bathroom like you would expect any normal human to use it, but instead they decided that at that moment, HR Giger was their muse, and poop was their medium.

So if you're never opened the restroom door, and realized that you would have to start cleaning by the light switch, I can understand why you might not be familiar with why they lock the bathrooms in some places. And even worse is that it doesn't happen just the once, it happens multiple times before management finally listens to the employee complaints and installs the lock.