This doesn't surprise me. They have had their butts kicked more than once for discriminating against black customers. One time, they picked on a group of black men who turned out to be secret service agents assigned to presidential protection. Big mistake on Denny's part.
My own experience with them came as part of a group of about 10 people from a karate school who were in the habit of stopping for a snack on the way home from an evening workout. The group included the owner of the school, instructors and older students. Ages ranged from late teens to early fifties. We were a quiet, well behaved group who brought business when the restaurant was nearly empty. However, rudeness from the staff, slow service and cold food made it obvious we weren't welcome. Eventually, we were told never to return. We switched to a nearby Village Inn where we were appreciated. I haven't had a good word to say about Denny's since.