Something which now looks suspiciously similar happened to me:
http://www.flyertalk.com/forum/delta...d-i-fight.html
Wondering now whether it wasn't for the same reason. I did not have an F ticket, paid or award -- I had been upgraded -- but! -- I was in 2C.
Supposedly, 2C should be just as good as 2B for FAM purposes... right? And if, say, the passenger in 2B had an actual F ticket... there we go.