I wonder if in those rules stations are required to talk over the beginning and end of songs
I've been rolling a theory around in my head lately about this. Back in the old days, like the 1930's, one side of a 78rpm record could hold around 3 minutes of music, give or take. Initially, radio wasn't allowed to play records (the record companies being worried they wouldn't be able to sell records, after all...). As such, there was no pressure from radio on how music should be formatted. The pressure came from the time limit. Songs got started quickly and ran right up to the end, sometimes coming to very abrupt endings, sometimes having nicely choreographed endings, but certainly never having anyplace that a DJ could possibly have jumped in and talked over the song.
Fast forward to the 1980's. "Singles" are still under four minutes in length, mostly as a matter of tradition, not technology. Virtually all of the 80's music on my empeg fades out at the end. I suspect this is a direct result of over-talking from the radio stations. If they're going to talk over your song, you might as well just end your song in a fade-out so they don't mangle the meat of the song.
I don't know if there's any explicit regulation on talk-overs, but there's plenty of evidence that they've been around for a long time. Certainly, you'll never hear talk-overs on jazz, classical, or college radio stations. I think it's just some bastard creation of the commercial radio stations.