No, America's segregated (and genocidal) past does not rationalize the evils of apartheid, nor do the evils of apartheid eclipse the past and present evils of racism here in the U.S. And there is no reason to single out the U.S., either. As you rightly point out, the U.K., Canada, and Australia are other favorite destinations for South African emigrants, a list that should also include Israel -- all countries with their fingers in the genocide pie. Coincidence?
Hard to fathom from a bar stool in Fort Lauderdale? It would seem that nowhere is "half a world away from the scene of the crime." In fact, to (mis)quote the old colonial proverb: "The Sun Never Sets on the Scene of the Crime."