Yes, it is a matter of personal responsibility, but it can also potentially affect the wallets of others - especially those paying their salaries. For instance, if I am the owner of the Steelers, and we had lost by a couple points in that game against the Chiefs. I could conceivably blame the loss on his hair, assuming they didn't convert on that drive, and what if that loss was the difference between making and not making the playoffs. "For want of a nail . . ."
Also, if he gets a chunk of scalp jerked out in the first half of a game and has to sit out the second half, as the owner who is losing the services of a player that I'm paying 7 figures for - I'd be pretty angry about his personal choice to keep his hair long. However, even if I am unhappy about it and feel it's losing me money, the players' union would no doubt interfere with any attempts I made at getting rid of him for that reason, and even if I could get rid of him, I then have to go find an equally talented guy, etc.
In sum, it is an issue of personal responsibility and all that, BUT as a professional football player you are signing a contract that means a whole lot of people's incomes are affected by you. I think those that are paying the millions should be able to make the reasonable demand that the hair at least remain tucked up under the helmet.
I actually think it's a tougher call as to whether to impose such a rule amongst the NCAA football ranks.