I think in the case of the modern leftist, their political views are so far out of touch with the average working American, that it's natural to consider holding those views to be a mental disorder.
Of course, that's just one way to look at it. If we take the extreme radical lefty whackiness out of the equation, then you're correct; holding certain political views does not mean someone has a mental disorder. But dayum Octavia, some of those views just don't even come close to the threshold of common sense.
