Nobody's forcing you to buy an XBox 360 controller, and several other kinds of controller should work fine. However Windows has drivers for it pre-installed, some other systems support it by default, and C2's Gamepad object uses the XBox 360 controller buttons as the standard button set which other controller buttons are mapped to, so it's generally the easiest controller to use.
Also note I think the wired controller is cheaper and works out of the box - using the wireless controller with a PC requires some driver installation IIRC.