i'd be interested in seeing the letter. it seems incongruous to charge someone with a violation of a law for failing to address a recall they reasonably might not have known about (aren't recall letters sent via regular, not certified, mail?).
that article seems to refer only to a dealer's responsibility; dealers are held to a higher standard of care than individual resellers. it also seems to draw a distinction between situations in which the seller knows or should know the recalled product is dangerous and fails to reasonably inform the buyer of the danger, and those situations where the open recall does not render the product dangerous. in the former, the seller might incur liability for the buyer's resulting injuries; in the latter, that outcome doesn't necessarily follow.
also, there's a difference between a prohibited sale and a seller's subsequent liability for the consequences of an uncompleted recall. a seller might be liable for injuries to a buyer resulting from the seller's failure to exercise reasonable care, but that would not bar the actual sale itself. the article deals with the dealers' liability for the buyers' injuries, not with the legality of any sales.
all that said, i agree that people should just get the recall done and quit trying to outsmart the mazda engineers. even if it does cost a few hp, what difference does it make? these are road cars, not F1 racers.