Do you mean devices with RFI-suppression components omitted to save cost? Those devices are subject to FCC part 15 rules just like devices manufactured domestically.
> from off-brand vendors
Again, who is supposed to enforce that these vendors comply with US laws? My point is that those laws are not enforced. The reason that look-alike Apple USB charger costs $3.99 is because it contains no RFI suppression parts and is probably made with toxic plastic.
It is the job of regulators to shut down firms importing this garbage. By failing to do so the regulators harm the people their regulations are supposed to protect. Meanwhile, US firms who comply with the rules can't sell their USB chargers for a competitive price so they go out of business.