It comes from when Hershey's made chocolate before milk trucks were refrigerated. The milk has to travel further in the US, and it would go off, but Hershey's would use the milk anyway, and that's where Hershey's developed their signature "sour vomit" flavour to their chocolate. It's been going on so long that Americans think that it what chocolate is supposed to taste like, until they get the good stuff from Europe.
Ahh interesting facts! Feel bad for them thinking thats what it should taste like...