As long as it doesn't bright up the area I don't see why it's such a big deal. Let people make memories how they want to, and you should worry about you.
Ummm, by definition, it does 'bright up the area'...it's a flash. It makes everything around it brighter.
Using a flash usually doesn't make the photo any better. it does make the area right in front of the camera nice and bright though. I seldom use the flash when taking photos in WDW. When we went to Cape Canaveral, we were told absolutely no flash photography. Okay, no problem. I disabled my flash and went happily on my way. Well, I overhead a 'gentleman' speaking to his wife, in not so very quiet tones..'Well, evidently she feels she doesn't need to follow the rules about taking photos....must be nice to be so special'. Well, I just turned to him and told him that photos were fine, just not flash photos..or hadn't he understood the tour guide??? He said that you couldn't take a decent photo inside without flash..so obviously I was using flash. I politely stood next to him, took a photo of some display, and then asked him he had seen any flash??? Ah, no, I didn't. Then I showed him the photo...very nicely done if I did say so myself. The poor man was so apologetic. He said he didn't realize you could do that...yada, yada, yada.
The vast majority of people have no idea that they can do away with their flash. Watch parents at plays or dance recitals....all usng flash from 40' away. Have to wonder how those shots are coming out.
But, on Haunted Mansion??? Or Spaceship Earth??? Or Ellen's Universe of Energy??? Nope. And I've had it happen on all those attractions. At Ellen's, the CM kept coming on over the PA, telling the person with the flash camera to put it away. He finally said that if the camera wasn't put away, it would be taken away at the end of the ride!!! That seemed to make an impression.
At Haunted Mansion??? There we were, at the Madam Leota area...floating head and all that. And of course, someone just has to start with the flash photography. After the third flash, everyone was yelling "No Flash Photography!!!!!!'