You have a taste of this with the EU. The EU consists of a number of member states, each with their own laws and law enforcement mechanisms. The EU itself provides a common framework for those laws. The EU is a bit different because it pretends that its not a Federal republic (and so avoids all that Federal level democracy, making do with fig-leaves).
If you want to see how this would work out in practice in the US just look at the laws and their enforcement with regard to marijuana. Its legal in a number of states but illegal at the Federal level so you could get problems if you transported a load of weed between states ("Intrastate commerce" is Federal jurisdication) or came to the attention of Federal law enforcement (such as a Border Patrol checkpoint). Enforcement of laws is hit and miss and unduly bureaucratic in California because local jurisdictions also have a say about what's legal to sell where. The result is that the industry bumps along avoiding situations where it could be regarded as problematical and the rest of us just wish the state licensing and enforcement people would just dissapear -- they're more trouble than they're worth. Mapped to encryption it will be a case of "you and who's army?" -- states don't have the manpower or expertise to get involved.
As for the 'child abuse' thing, we've moved on from pizza parlors to on-line upscale furniture vendors as the 'conspiracy theory du jour'. There's some seriously weird people out there -- and a lot of them seem to be in Congress.