I know why. EA/HB didn't include proper alpha maps on many of the generic faces (plyr122.fsh, plyr085.fsh, etc).
You can fix it by downloading the cyberface sweat pack from 07 that adds sweat to all the default players included in 07.
http://www.nba-live.com/download/pafile ... le&id=1216
Then you can open it up in bigGUI and select all the plyrXXX.fsh files and export them all at once into your 08 sgsm directory. It will get rid of the shine and give them a sweatty appearance.
There are several nba players who also don't have alphas and will probably end up with shiny heads. Keep the cyberface sweat pack handy and whenever you run across one, extract that players face like you did with the generic faces.