Some FM HD music stations sound like mono?

I'm sure some are. Most stations do not care about their HD2 stream, running it only because some bigwig from corporate sent out a memo ordering them to. Or it could be that they're doing it on purpose to counter the artifacts, which I suspect with some early wiiiiiide stereo mixes would be particularly heinous.

All that being said, an awful lot of oldies ARE in mono. You sure that's not what's going on?

It's also possible that whoever encoded the music may have thought all music before 1960 was in mono and thought they'd save a few bytes by not encoding them stereo. The files on a typical station's playout computer are often a random collection of PCM files, MP2 and MP3 files at varying bit rates, and songs that DJs who don't work there any more encoded years ago.