Interesting passage from a book I'm reading (emphasis mine):
"Once commercial radio took off in 1922, it would take the record industry, publishers, and the union decades to allow radio DJs to legally play records on the air. Rather than embrace radio as a promotional vehicle, record companies placed warnings on record labels, forbidding their broadcast. The musician's union struck for almost a year to protest the practice of playing records on the radio.-- Stephen Webber, "The RIse of the Radio DJ", DJ Skills: The Essential Guide to Mixing & Scratching
"The American Society of Composers and Publishers (ASCAP) forbade radio from playing any songs written by their members. In response, the national Association of Broadcasters (NAB) formed Broadcast Music International (BMI), which signed up many young country ("hillbilly"), folk, and African-American songwriters and artists to write and record records that could be played on the radio. This move had a significant impact on the development of popular music on the airwaves, giving an early boost to musical styles and musicians who may have been kept off the radio entirely, if radio's commercial potential had been known."