Have you hear about customs?

None of the electronic widgets I've bought stateside from overseas sources have ever shown any evidence that they've ever been actually-inspected by US Customs.

How would they know that an item is fraudulently marked if they never look?

For all things you buy in local shop, responsible owners of shop. If you buy anything abroad, this is your own problem and it is not relevant to subject.

I note that none of these entities are the "customs" that I have heard about.

You just don't know, how work defense of internal market in typical country.

1. Organizations listed in subject (NIST, FCC, deputies from tech companies) constantly create or even invent methods to check products quality and to enforce penalties for offenders, and propose regulations to approve by parliament.

2. Parliament make juridical documents and approve budgets for 1 (and 3,4 when need).

3. Customs limit penetration of abroad subjects to internal market.

4. Police, courts, deal with internal offenders, or with abroad offenders managed to infiltrate through customs to internal market.

In real life, local shop become responsible when sell products from abroad, and regulations limited possibilities to create local shops for foreigners.

Unfortunately, life is constantly changed, technologies constantly grow, so old regulations eventually become obsolete, so all these things work in endless loop.

It doesn't fucking matter how things "work defense of internal market in typical country".

Typical doesn't fucking matter here. This is a fucking US fucking policy, and the only fucking place it fucking matters to a fucking consumer in the fucking US.

For fuck's sake! Fuck.