When the British Empire was in full flower, colonial administrators didn't try to force the natives of their colonies to become British. Now when the citizens of those now-independent nations migrate to Britain, their behavior is often offensive to native Brits. Grooming (rape) gangs, cousin marriage, hijabs, etc. don't sit well with the English, Scots, and Welsh.
By contrast, US colonial administrators have largely imposed US laws and customs on our colonies and their natives. Those natives haven't always loved the experience, but when they move to the US they often fit in better here than the UK's ex-colonials do there.
Full disclosure: the DrsC lived and worked for a year as civilians in a US colony in Asia - Guam. We experienced the occasional awkwardness that resulted when Asian customs encountered US laws. However it worked reasonably well, several thousand from Guam thrive here in "the States."
