Aren't you always making moral judgments on non-political videos too? Which videos are "adult", which videos are scams, which videos are inappropriate, etc...
So I think the distinction here is that "what is porn?" (again, a deep question that the supreme court has addressed, considering how much nudity there is in art) does have a non-moral answer, even if it's arbitrary (such as whatever arouses 10% of people).
The other distinction is that with political speech is that if you screw up that line you hamper criticism of authoritarian regimes (for example). If you set the bar too tight on your porn filter, it's much less likely you stifling political progress.
