With the money they[AB&B] make, could they not afford some kind of Zaggat-like roving rating staff who go around unannounced once per year to rate these places[1]. Not necessarily stay there, but go in and look around for basics. This would be the "hard" rating and then you could also have the guest generated "soft" ratings as well.

It need not be mandatory, but optional and eventually those not opting in are evaluated more suspiciously.

[1] places that rent out at least 100 days per year or some such cutoff.