Dumb question regarding adult content filtering

It just suddenly occurred to me:

All of the services like NetNanny out there, I wonder why they don't simply search for the string "18 USC 2257 Record-keeping Compliance Statement" and use that as a way to filter out adult websites?

The only "casualties" would be First Amendment-related legal information sites, anything else with a 2257 statement is, by definition, going to be something that has or potentially could have adult content.

I remember two years ago, we discovered that one of the major child filtering services had none of our sites in their database. Half a dozen emails answered by worthless drones in their email response center went nowhere to getting us added. Only after I threatened to go to CNN and report that a major filtering service refused to help us add our adult websites to its database did someone magically take an interest and fix their broken submission page and manually add our sites.

Spidering for the 2257 phrase would pretty much eliminate most of the problem.

Other than the example shown, is it worth it to have multiple filters, or is it just overkill?