I think saving the PNG map with 64-65 colors would solve the bg issue, and make both type of people happy.
To be honest, you'd never catch me playing League BnG on pre-made maps anyway, only funners.
They are so easy to abuse it's insane, especially the symmetrical ones!
That's why I prefer the classic BnG .bit maps, all you need to do is reseed them, change the texture, and maybe quickly edit a few hides and voila, new nice map! You can't do this with the pre-made maps either lol.
Good to know though, if I have to host/stream a BnG match where someone picked those maps, I can at least do it right so we're all happy!