Why do US presidents always dye their hair gray?

I've had a few silver hairs since my early 20s (I still refuse to call them gray haha) it is genetic they tell me, my father had about the same thing. As for the presidents though I'll suggest a different spin for it that has been true for a few of the previous presidents, although I don't know for sure about Obama:

It is not that they are dying their hair gray, but that at some point in their presidency they get too busy to keep dying out the gray in their hair as they did in their campaign. This makes a good bit of sense, saves them some time in their day and really doesn't matter once they are elected.