I asked on IRC, and got this answer: The archive (Packages) and individual .debs can disagree on Priority. It's mostly a field that has no meaning these days.
I'm not 100% sure how to interpret that. Are different mirrors giving out different Packages files with different Priority settings? Someone who's affected by the missing capabilities on ping might want to investigate more closely, or add some info to bug #780721. Adrian talked about libcap2-bin being changed to "Priotity: important" at some point in the future (relative to 2016), so maybe that has already happened. I'm not sure how to *find out* whether that has happened, since we can't even figure out how to view the actual Priority. Or, if the Priority "mostly ... has no meaning", then whatever was stopping Adrian from acting in 2016 might no longer be a roadblock.
