This is inspired by Sgt. Mom’s Borderline Radio post.

I was originally going to ask the call sign of Wolfman Jack’s radio station from American Graffiti. But that alone would be too easy. So, in American Graffiti, George Lucas portrayed Wolfman Jack’s studio as just on the outskirts of Modesto, CA. Name the famous legislation which made this an impossibility (as well as said call sign).

The Answer! I knew this was too easy. Reader M. Thompson has it down (see comments).