I think WM has minimum advertised price agreements with their retailers. I asked Moosejaw why WM wasn't part of their recent 20% off a regular priced item sale and they told me their agreement with WM prevented it.
A few months back I got a WM Bristlecone from Backcountry at 20% off. They has a 20% off sale bit the bag wasn't discounted. I called and they gave me the discount. They handled the entire transaction as a one off.