Pretty much the same reason why Americans think it's bad for health. It has a scary-sounding name (it's also called "MSG" in Korea), it sounds like a "chemical", and you can find it in ingredient lists of all popular junk foods - most notably instant ramen, which is basically an MSG+salt solution masquerading as noodles.
Give something in food an exotic name, and people will invent reason to fear it, like all those people demanding gluten-free bread.
Besides, if "racism" is the explanation behind it, how are you going to explain the immense popularity of Chinese food everywhere in America?