Just make a new smart playlist whose "Kind" contains the word "Protected." 'Cause only and all things from the iTMS are a "Protected" format. Either Protected AAC or Protected MPEG.
That's the best fix I have. It's weird that there's nothing like that in the iTunes preferences...
And I don't know for sure, but *maybe* deleting your iTunes preference files could solve it? (spotlight for com.apple.itunes.plist; it'll be in your Library/Preferences folder)