I don't think Pantheon's AS nerf is really about the dragon. On PBE, they already had fixed the dragon attack a spell so that Pantheon (and Fiora, etc) couldn't block the damage. No need to nerf AS to repair that.
I think it's about how many blocks he gets while clearing the jungle. Back when sustain wasn't a thing, this didn't matter because all junglers were full health all the time. I'd bet that Riot thinks Pantheon's jungle sustain ought to be more like other light fighters in the jungle, and that's why they nerfed his AS.
I'd of course love to hear a clear explanation from Riot on this one.