An airline or UPS can absolutely say “we don’t like the contents of what you are shipping so we refuse to transport it”. UPS will not ship marijuana, even for medical purposes, even in Canada where it is entirely legal. It won’t ship watches over $500 or ammunition.
I disagree with turning platforms into common carriers. As I wrote, I think this misses the issue: a protocol should be a common carrier, but a specific provider has no obligation to host things it doesn’t want to or do not make sense for certain territories.