Its a good point. Why should a striker similarly equiped to a defender (i.e. two handed weapon) have a significantly higher ac? Ac is meant to be part of the defenders bread and butter, where strikers sacrifice defence for attack and mobility. But here we have a situation where a Striker boasts one of the highest ac achieveable in the game.
It got corrected, now it appears they are uncorrecting.
As to the whole "Avengers dont have to take the feat" argument. Well, yea, but by that logic every class should have access to a feat which gives a +2 ac on an already adequate ac given role. Why should avengers alone get this privelege? (and no, the whole "avengers need it" bit doesnt wash. It doesnt apply to other melee strikers, who must put themselves in peril, so why should the avenger get special treatment).
They don't get any special treatment. A ranger that wants Defender level AC can just opt to invest 3 feats and wear plate.
A Barbarian can take Hide Armor Expertise.
A Rogue can invest in hide armor and focus on dexterity.
However, a defender doesn't need to invest said feats: his AC is usually pretty high right out of the box: just like you can build a fighter that deals striker level damage, you can create an avenger, barbarian, rogue or wizard that can reach defender-level AC, provided you are willing to spend a fair chunk of your (limited) resources.
If a striker spends those 2 feats for 3, 4 points of AC, he's falling behind in terms of damage: as you said, an Avenger's AC is more than adequate for a striker. Thus, as long as the defender in the group does his job, a striker that wants to be the best at his role should be using his feats and powers to reinforce his schticks ( damage and mobility ), rather than waste his resources to improve his AC. Vice versa, if a group lacks a defender, the Avenger/Barbarian/Rogue/whatever can still opt to improve his defenses.
Sure, you can argue that if everyone has defender-level AC then the defenders become useless; however, the party is also losing a fair chunk of battlefield control: sure, that Avenger might have an AC that's as high as the fighter's, but what will he do when all the opponents on the battlefield focus-fire him and there's no fighter or paladin to take the monsters away from him? A good defender doesn't just grab as many monsters as he can and take all the heat, a good defender knows how to spread the monsters' damage around.