On page 16 of the Operations Manual under Attack Land Units and/or Airfields it says:
For each of your air units and sea units participating in the attack (not land units), decide whether it will attack the enemy’s land units or airfields.
The “(not land units)” implies that land units don’t have the option of attacking airfields. I agree that the wording isn’t the clearest. I’m trying to get it added to the FAQ, but I think the move of the forums to Gleemax is taking up a lot of AH’s resources right now.
Thanks, Krieghund! I agree - the wording is poor. It could also be interpreted as “the sea and air units have the option of participating in the attacK while the land units MUST attack to resolve current combat” which is the mistake I made. That being said, it would be pretty silly if 2 infantry units could damage an airfield every single turn.
Thanks for the clarification.