I too like Black Bear Diners and will usually pick them regardless of their participation. I have found many come and go from the Rewards Networks program. I have noticed and presume that happens with all other chains as well (i.e. Jimboy's, Togo's, etc.). I don't know if it's a cost cutting effort on the part of the property or a lack of benefits to the property. It may even be related to a change in CC processor not connected to RN.
The response from RN is appropriate. You must check specific properties for participation (repeatedly). Not allowing a property to advertise because others do not participate would not be fair to those that do. Forcing all properties to participate just because one (or more) do is not fair either. Some of these are franchises that may have no direct relationship to others and thinking it's an all or nothing proposal just doesn't work.
Now a direction I would like to see RN head is when new properties join RN and I've dined at sister properties or a property that I have previously dined at leaves the program, an email should go out to keep me informed.