1. The government has a public interest is assuring the age of majority of performers depicted in explicit sexual content. I would expect that §225
6 would withstand scrutiny.
2. The government may not be able to regulate adult content's location ** on the internet [added]but they don't now -- they do not prohibit internet use of certain domains for pornographic content as an example.
I don't see this as a good thing if the ruling goes either way. If §2257 was struck down Congress most likely would see fit to modify the law to suit the Court.
What I think would be fair would be to require valid identification at the time of production for the record and that is it -- the rest is just bullshit and an attempt to preempt otherwise lawful adult expression
Burden of proof
should be on the government -- §§ 2256 - 2257A is in USC Title 18 --
that is criminal code.