It's a Wanker Tax
Most folks (in NC) who want porn won't figure out how to disable it, live too far from a state line, or simply don't feel it's worth $20 to drive six hours to buy an unlocked computer. They will pay the $20 to keep their access going. Multiply that by the legions of porn consumers, and you've created a nice revenue stream, an indirect Porn Tax.
What drives politicians? Money!
Hey NC legislators, why don't you give porn a break and instead make life hard on the plethora of tobacco growers in your fair state? Let's take relative harm into account here! After all, porn doesn't cause cancer, only blindness.