But they had to follow the frequency-of-play rules you listed above with regard to Rush's Atlantic material, whereas they had the freedom to play as many Mercury-era tracks in a row as they wished.
My guess about that is that the labels have a certain number of waivers they can use at a time. I believe they also have to be renewed regularly (maybe weekly). Since this tribute channel was for a month, perhaps that was more than someone was willing to approve.