That's not really true. Its one of the many, many actions that is not specifically listed in the rules as a red card offense, so the referee has to make a case-by-case determination whether the action rises to the level of "violent conduct" or not. I'm pretty sure head butting and stuff like that is exactly the same.Actually, hair pulling isn't against the rules.
The question is why it wasn't deemed violent conduct, especially when there are precedents like this one where the hair pull was deemed violent conduct (wasn't seen at the time but Huth got a retrospective red and three match ban). Huth just yanks Fellaini's hair back, Romero pulled Cucurella all the way to the ground.
View: https://www.youtube.com/watch?v=a91mrDs6dSE