Maybe Shawn Crawford is an extreme example, but all Olympic sprinters have ripped, cut, buff upper bodies. You don’t see this in the longer distances; 800m and longer runners tend towards long & lean body types.
Why do you need arm strength to sprint?
Or is just that the steroids give you muscles everywhere, whether you need them or not? 
Because in a short sprint, every gazillionth of a second counts. If they can use their arm muscles to generate driving force to yank themselves forward, it makes them that much faster.
Not so useful in longer distances.
Sprinters still need to look good in product endorsements. And, uh, there really oughta be a good groupie joke in here somewhere.
There was a previous thread on this very recently.
Because proper leg training will make your arms big.
The ripped look comes from having very low bodyfat. Even normal-sized arm muscles will look fairly ripped if they have good muscle tone and very low bodyfat. And sprinters certainly seem to be extremely bodyfat-deficient, so…