They don't actually love America. They love corn fields, and white people, that "purple mountains majesty" stuff. They love the romanticized version of America that all the songs are about and that was kinda pushed on us as children (especially if you went through school while the Cold War was still on). The cognitive dissonance would hurt too much for them to admit that the country they love so much doesn't actually exist, and never has. So when we criticize the country we do have, and insist that there's room for improvement...well, America is already near perfect so we must just be unpatriotic, right?