Akali's passive does not work against structures. EDIT: It works as of 6.24. My apologies!
Ziggs' W change was put there to bring him back into the meta, due to being pushed out for more favorable picks that could push AND take towers.
Tristana's E change was put there due to the fact that she had little to not way to synergize attack speed with more attack speed from her Q. She built BotRK, Shiv/Rapidfire, etc, when they made the change. Now that she builds equally as heavy attack speed items early and builds more ad late, she still needs the E change for the early tower clears.
Vayne's Q was given to her due to the fact that not being able to Tumble towers felt clunky and was normally used as an auto attack reset.. once every 3-6 seconds. It felt bad, looked bad, etc, so the change was made.
Janna's unique mechanic of being able to put a shield of wind over a tower is a piece of her kit that forces her to choose between shielding herself, an ally, or saving an objective. (That may give 400 global gold to the enemy team if it's the first tower taken!)
Also, Janna's shield isn't only physical. :) It's all damage types, regardless of application. I believe it's stated as, "shield that absorbs x damage" in the tooltip, as well?