Being nostalgic for certain elements of a time in history doesn't make you nostalgic for all the other elements of that society.
But see, no one ever SAYS that. No one ever says "I'm glad we have civil rights, etc. but there were some things about the 50s I miss." They go whole hog and say how much better things were in the 50s (or whatever rose-colored time period they're looking at), full stop. And that's when others feel compelled to say look, dude, you may be fooling yourself, but we know better.