The fact of the matter is that it is both ridiculous and uneconomical to expect the US government to conduct investigation into every website that it suspects of containing pornographic material involving minors. After all, many women naturally look younger than their age (I'm one of them) and this can complicate matters further when the porn itself is actually advertised as having "teen" actresses and being lolita.
However, as a result of book keeping records being required pursuant to §2257, these websites can easily and expediently exonerate themselves by providing documentation or proof of an actress/model's age by providing sources for the material. So, not only does this law protect many providers of pornography from undue suspicion, but it allows investigators to conduct their searches where they matter regarding the increasing, unregulated rise of underage actors in pornography.
Child prostitution and pornography has been running rampant in states bordering the US-Mexico border (such as California and Texas) because minors are easily smuggled in from Mexico and forced into these trades like modern day slaves. When it comes to the Internet, tracking this activity alone is an incredible task; especially when it comes to finding it's source. This isn't about censorship so much as making the process of investigating these crimes easier for all involved.
Personally, I also fail to see why Rotten.com should be exempt from the same requirements that normal businesses online must adhere to. All that this law asks for is for pornography websites to take the responsibility of regulating themselves. That isn't censorship, but protecting the rights of whomever may be involved in the material being posted; especially in the consideration of minors.