Mayonnaise on hamburgers

I would hazard a guess that this is about the mayo forming a barrier to prevent burger juices from sogging out the bun.

I’m willing to try this once. But if I still loathe mayo-burgers, you will hear about it.

Just like your second paragraph but the exact opposite. If I was to make a burger, it would be mayo free. That doesn’t apply to fast-food burgers, which are pretty nasty already and, therefore, I’d eat them however they’re served (which is usually with mayo).