The carribean med schools, as a group, have relatively low standards and were set up with the idea of allowing those US students who did not get into US med school have their 'second chance'. It has nothing to do with American snobbery.
There is a difference between carribean schools and non-US schools that educate doctors for their native countries---i.e Uk med schools, or med schools in Canada would be considered "foreign" med schools yet do not suffer the stigma as Carribean schools (although I am unsure about how they compare during matching).
Many of the carribean schools geared toward US students were not set up to train doctors in their respective countries. They were setup as for-profit institution to fulfill the 'spillover' of the high demand for medical seats. Some of the schools there *are* no better than diploma mills. That is why they have that stigma.