Under California law, it is unlawful to advertise something as being “Made In The USA” if it was not, in fact, “Made In The USA”.
The California Business and Professions Code §17533.7 says:
It is unlawful for any person, firm, corporation or
association to sell or offer for sale in this State any merchandise
on which merchandise or on its container there appears the words
“Made in U.S.A.” “Made in America,” “U. S.A.,” or similar words when
the merchandise or any article, unit, or part thereof, has been
entirely or substantially made, manufactured, or produced outside of
the United States.
I merely point this out because each state has a whole set of advertising laws that one must contend with if one is selling products in that state (and maddeningly, they will be different in each state). This serves as a good example of the kinds of things that might become an issue if not addressed early on.
That said, as many products have parts that are assembled outside of the United States and then put into final form within the country, it is hard to tell, without reading the case law, what constitutes a product being “Made In The USA”.