You see, back then, most people thought it meant the hybrid animal like the jack ass. The definition you're thinking of was an insider term shared within Rhythem & Blues cultue which eventually spilled over into the type of Rock and Roll which preceeded Rockabilly. Once the Squares caught onto what it meant, as far as the animal's species name was abused to define someone's behind, communities made them take down such signs. Refusal to comply was sometimes met with lynch mobs, in remoter parts of the country. Luckilly, it never resulted in actual punishment, since any remaining resistance had enough sense to give in and comply