First of all, they're right when they say that classic movies like E.T. and STAR WARS and Casablanca are the property of our culture and society. I speak of this in my essay on fanfiction.
But - as I say in that essay - they belong to society so they may be used.
How many exterminators have cartoon mascots with deerstalker hats and/or magnifying glasses? How many products and services call themselves Sir [fill in the blank]-a-lot? What percentage by time of that SOUTH PARK episode was scene-by-scene parody of RAIDERS OF THE LOST ARK?
Yes, such things belong to the people. But what Matt and Trey have missed is that George and Steven are the people too.