What I've heard that dark stations do when they're searching for buyers is go on for one day a year to keep their licenses.

Could some of those dark AMs be hoping to get FM translators? They might be thinking that will work to revitalize them, running the AM station but selling the FM.

I agree about Part-15 on AM, as far as space to broadcast, and range, AM has it, at least in my city. Here the FM band is brimming with stations, there are some spaces, but there's so much intermodulation from full power stations and DTV interference hash, that it's hard to do much with a low power signal.

AM however, has a dozen frequencies in my area where a P-15 neighborhood station could run on, and it's far enough from the VHF bands that it doesn't really have noise problems from stations there.