This "challenging for the ball" thing is only really relevant if the action is considered to be part of the challenge (like if a player is going in with their studs up but clearly trying to play the ball). Hair grabbing is generally considered a "non-football" action regardless of the context. The excessive force is also not especially relevant if it's considered to be a deliberate act of aggression.
Anyway, leaving all this aside, can we agree it's a weird thing to do? There's a lot of people defending something that I don't think I've ever seen another footballer do while challenging for the ball in the air.
Yes, because they are focused on heading the ball. Which is what Keane is doing.
The amount of people who think Keane can both barely put one foot and front of the other, and at the same time jump and win a header while pulling a guy's hair, is pretty astounding.
At any rate, it's not worth continuing to discuss. I'm not usually one who believes in bias in officiating, perhaps this wasn't a matter of judgment so set it aside. But the amount of judgment calls we're on the wrong side of seems a bit high.