It's not going to be a clear-cut case IMHO. Since it does not look like UA promised anything other than "significant savings", they could offer discount zone fares below walk-up fares and get away with it. It doesn't look like they promise fares to be cheaper than advance booking fares that might be available. As long as Silver Wings can book a zone fare ticket, I'm not sure what the case is. UA's hope is that the Silver Wings members go away (and the demographics are in UA's favor) by streching it out.