Look closer, sprinters have muscular everything...
People tend to lump all different types of running into one category but it's more complicated than that.
Marathon runners run long distances, within an aerobic heart range (ex 133 < 152 bpm for a 30yr male), maximizing distance by decreasing load as much as possible. Typically, if you do a dedicated endurance running regiment your body will favor Type I (slow twitch) muscle mass which uses energy (oxygen, glycogen) more efficiently and reduce Type II (fast twitch) muscle mass which trades efficiency for power. That's why experienced marathon runners will typically appear very skinny.
Sprinters are the opposite. Sprinting requires exercise to be pushed into the aerobic ranges up the V02 max (ex 152 < 190 bpm for a 30yr male). The body has a limited amount of time it can maintain an anaerobic intensity due to the impurities that anaerobic metabolism create. To increase the intensity and duration you need more Type II (fast twitch) muscle mass and an increased ability to process the impurities created by anaerobic stress.
That's why you can hit the 'runners high' during an aerobic workout where you feel like you can keep going forever and you get the 'muscle burn' from doing anaerobic workouts.
So why are sprinters so muscular?
First, increase your anaerobic range to be higher by doing HIIT (High Intensity Interval Training) workouts. Basically, do short hard bursts for of activity (like sprinting) for a few minutes followed by recovery periods to allow your body to recover from the anaerobic stress. The rule is, the harder you push, the harder you will be able to push.
Second, here's where distinguishing between marathon runners and sprinters will start to make sense. HIIT training (including sprinting) uses your whole body. Make no mistake, high impact, high intensity exercises work your core and upper body as well as your lower body. Unlike endurance running, it's actually beneficial to focus on all muscle groups as long as you're not just putting on unnecessary bulk.
Third, Type II muscle burns lots of energy and I mean a lot. Not only do sprinters have a lot of Type II mass from head to toe, they also have highly optimized circulatory systems. The high energy burn and increased blood flow lead to a very low body fat percentage giving a very defined 'cut' look.
In conclusion, Type II muscle mass and upper body strength is bad for an endurance runner because it adds weight and unnecessarily consumes more energy. But, for a sprinter Type II upper body mass is generally a result of working out and also a good counter to balance the force being exerted by the lower body.