Why does Rammus's passive give AD?

So I just got rammus in ARAM and saw that his passive gives bonus AD which is really weird because this has zero synergy with any aspect of his kit. All of his abilities scale off of Armor or Ability power (or a combination of the two) and he has no bonus to attack speed or even just an ability with an auto attack reset to take advantage of the increased attack damage.

The passive isn't necessarily bad by itself but it's just so out of place with the way his kit works and the theme of the character in general.