If you ask me obfuscating key information from players is never fair or good game design. I recently got tricked into picking alistar as the sole frontline damage sponge/initiator for my team only to find out in game he takes 10% increased damage. How was I supposed to know that this sub 50% winrate tank support is not supposed to be played as tank?
He's still supposed to build tank for sure; that's the point. He takes 10% increased damage because his kit is built to be a powerful initiator in 5v5 fights and that is far more accessible in an ARAM situation. I took a peek at his win rate in ARAM, and its been oscillating a half a percent under 50% for several patches now on the tank build. You are supposed to play him as a tank. If you read that he takes an additional 10% damage, and that tells you to NOT play him as a tank, then you're going to have a far worse time in that game then if you just built tank items on him.