Yeah, I take equal issue with flying both a Confederate and Mexican flag in the typical context they're flown(which is usually, F YOU AMERICA). A lot of Southerners hate, hate, hate, Yankees. That's mostly what it's about. It's actually far less about racism and slavery, and much more about asserting true Southern roots.
Most people flying Mexican flags are up to the same thing. I'm from Mexico, have ZERO interest in integrating into American society. But I expect you to foot a $10,000/year bill to provide my numerous children with bi-lingual education even though I pay about $2,500 in taxes per year.
Can't say I've ever seen someone fly a French flag outside of a French restaurant. I've never had a problem with people flying Israeli or Canadian flags because I've never met people doing it specifically to disrespect Americans. I probably would have been fine with Mexican flag flying a good 40 years ago, because it was probably truly about pride instead of snide anit-Americanism.