The OEM guys selling the same bikes as pedelecs in the EU are under strict restrictions to be able to prove that what they are selling conforms and penalties if they can't. So far, they are the same bikes as here just with different controller parameters. So for sure, they have a lot at risk and are lawyered up.
The kit bike guys like you said, are small fry in comparison and are figuring it out as they go. There really aren't significant any fines or penalties for selling anything that doesn't conform and they are trying to make a buck selling what people want to buy. I don't know if they would have any liability in a lawsuit if a lawyer figured out what had been sold as legal, actually wasn't. I'm sure it will come up at some point.
And the chinese could care less, they'll tell you what you need to hear.
Speed limits are the most rational way to control speed, since there is zero enforcement either way, it doesn't really matter. Almost all trails already have speed limits in their code already. In my talks with my local parks dept, they did tell me that there are power limits placed on vehicles to limit their liablility in an accident. A speed limit isn't as effective at that.