Such laws or rules most likely would be on a state-by-state basis. As an example, a rule in Minnesota (and the next part which can be accessed with the arrow in the upper right of the web page) requires that "all electrical equipment, including material, fittings, devices, apparatus, fixtures, appliances, and utilization equipment, used as part of, or in connection with, an electrical installation shall be listed and labeled by a testing laboratory."
This rule only forbids use of unlisted cheater plugs, not sale. I have not found a law or rule that forbids selling them.
Searching the usual places online, I see it is possible to find 3 prong to 2 prong adapters that are UL listed.
I will add that essentially the same skills are needed to figure out whether a cheater plug is more or less safe to use in a certain receptacle as would be needed to replace the two-prong receptacle with a three-prong one, and a properly installed three-prong receptacle is going to be safer.