Hair may increase the air resistance and weight and hinder running; given this, why do some sprinters keep their hair?
I'll assume you that you have competitive swimming in mind when thought of this. The main difference between swimming and sprinting is that a swimmer's body is mostly horizontal throughout the race while a sprinter is running upright. This makes a shaved head or a cap on a swimmer more valuable because they are leading with their head, cutting through the water resistance with it (well, their hands do a lot of the lead work but the head is important).
Also, think about how when you are swimming, you can feel the water flow down your entire body from head to toe. In comparison, when you are running, the entire front of your body cuts into the air resistance, but only "flows" the few inches that are the width of your sides. So basically, someone shaving their head or body for a sprint wouldn't hurt...but it would only help a very miniscule amount, not enough to warrant shaving your head (unless you need an excuse to try it out and see what it looks like).