Mathematics ("the systematic study of quantity, structure, space, and change") and computer science are both formal sciences (http://en.wikipedia.org/wiki/Formal_science). You can always broaden the definition of mathematics to encompass all formal sciences but it's useful in practice to be able to differentiate them: a mathematician is a different thing from a computer scientist, even if they both spend all day pushing symbols around inside imaginary formal systems instead of predicting and measuring the physical universe.