I don't quite understand why in the San Francisco Airport, when people land, the sign says, "No pornography is allowed to be brought into the US", when you can leave the airport, drive 15 minutes to an adult video shop and put in coins and watch all you want, or go to a hotel 10 minutes away and also watch hundreds of pornography videos for $9.99 each. 

Also, people can go home, and order pornography subscriptions either from a US company or from a foreign company using the internet.

**What law does this sign refer to?**