You are probably right. It's another non-sense discriminatory reason for something over something else, like in other regions where they claim that piano-accordions are for women and button-accordions are for men...Probably more about middle class snobbery - pianos were "in" but button accordions were for working class immigrants and folksie bums.
In any case, such discrimination may well be the cause for button accordions being in short supply in America, compared to piano accordions.