Isn't Crea the official boss...? My guess is Amuso is hated for what he and Casso did to the Lucchese family, so I doubt the capos and soldiers are eager to give him anything, even less take any sort of orders from him.
Apparently he isn't very well liked on the streets and was "invited" to step down as official boss but declined.
He still remains the official boss and Steve Crea is still acting boss/official underboss.
