It's not just spelling of place names, it's also pronunciation: in both North and South Carolina there is a city called "Beaufort", but it is pronounced differently in each state:
South Carolina : Byoo-furt
North Carolina: Boh-furt
And both pronouncements are wrong, considering it’s a French word.
Why can’t no single English speaking person (wether UK or US) pronounce it as it should?
Both countries like to use French words but not the pronunciation, God forbid!
You really don't get it, do you?
"Des Moines", a city in Iowa, U.S.A., is no longer a French name. It's an American name. And as such, the American speakers are free to butcher, bastardize and otherwise mangle it however they see fit.
We don't have a gov't. agency telling us how we shall spell and pronounce our names.
What about those multiple Swiss names someone posted upthread here? Would you have them select one and call that the "proper" name and ban the others?
Your view of how language works seems quite constrained and restricted. Maybe it's because of where you live.