People are ignorant of other cultures. They think that America is the best of the best, so if you choose something else you're somehow doing something bad. I think it would benefit these same people to live overseas for a while...the USA is not the only good country in the world. Just my opinion.
While this is true for some people, I think that is a vast oversimplification of why people view it as a second choice. Just look at how many people go abroad for undergrad and have an amazing experience (i'm talking about undergrads of all career paths)
Its hard to move overseas, and a lot of people have friends, families, some even husbands, children, and many have pets, all of which tie them to the US. It also tends to be more expensive due to many having an extra year, some not being accredited, and having to either ship things overseas or have to buy all new things. for many these reasons, plus more, for some people going overseas is a second option if they can get into a US school.
That being said, its wrong/presumptuous of people to assume that because an overseas school is a second choice for them, it is for everyone else as well. i think that that is more often the problem than what you stated. People often just look at the end product, that is a vet license, rather than the experiences that are acquired while at vet school, forgetting that going abroad has a lot to offer.
In all of the people I have talked to who would prefer going to a US school (at my school and at interviews), I have never heard someone imply its because the US is better/other countries are inferior. I haven't heard of people thinking that people with a foreign veterinary degree are not as good vets.
The desire to stay with whats familiar may limit a persons world experience, but shouldn't be viewed of as ignorance or intolerance.