It is, because it is. An example we've already seen is the use of q to designate an angle, the use of r to designate density, and now the use of s to designate distance. Don't fight it, that would be a losing battle. Now I must digress for a moment to keep the purists off my back. Velocity is not the same thing as speed .
Speed is a scalar quantity and has only magnitude, or size, associated with it.
Velocity is a vector quantity and has both magnitude and direction. Thus a vector requires two numbers to specify it while a scalar requires but one. To prevent appearing gauche, don't jump to correct others who use speed and velocity interchangeably. Even physicists get sloppy at times with these words. Use it below.