[Jaydelart]

I think part of the issue has to do with emphasis of the unusual or negative.
As an American, you probably aren't used to seeing a "No Foreigners" sign, so finding one (or a few) will have a pretty significant impact on your experience in the country -- and, by extension, your view of the country... Especially, if you are led there by unrealistic expectations and are from an immensely different culture. This argument, however, does not excuse the existence of such signs.

This is just my theory.