A few months ago, I noticed that they started tagging M11 and other discounted rates with this. It appears that they are somehow making it a "Mandatory Preference" thus the way it is appearing. (Mine usually says, "King Bed, Nonsmoking Room Guranteed. Portable Crib, Non-Commissionable Rate has been noted please inquire at check in.")
My gut is that this was done to catch folks trying to use certificates (EEO, etc.) with discount non-public rates. I hate it because it screws up my preferences (aka. Refrigerator/Pillows).