Why'd they give Sejuani's old passive to Malphite?

They realized that taking no damage at all from the first attack wasn't fair even after they made her ultimate single-target instead of an AoE stun.

So what was the idea behind giving basically the same thing to a champion they didn't take the AoE away from? Especially since it's on top of a shield and has a lower cooldown.

He's enough of an anti-AD from getting bonus armor passively and slowing AS. Giving him double his armor against any AD attempt at poke or any attempt to finish him off is a little bit much, on top of all the other buffs they gave him for the mini rework.