This is an urban legend. Safety defects have to be remedied by the manufacturer for a period of 10 years, but that remedy doesn't have to involve replacement parts.

https://www.nhtsa.gov/interpretations/timereplcepartpollak12...

I agree, looks like you are correct. It seems that it is just one of those things that manufacturers have agreed to do voluntarily, in the absence of a specific law. I imagine they have calculated that the loss of goodwill from abandoning a product quickly would outweigh the cost savings (especially since there is so much sharing of parts that keeping a few specialty components on hand is not going to move the needle much).