It's Valve. FPS games are their bread-and-butter. Real ones, not aim-assisted console ports. If the trackpads were just fancy joysticks, I think they'd have just stuck a pair of thumbsticks on the thing. I figure they went into this with the default assumption that the trackpads were pointing-devices and then went crazy with the weird haptic-feedback system to try and make them "feel right" as thumbsticks for games that need that. And since the haptic-feedback system is driven by code and not springs, it can be turned off for pointing-device usage.
I mean, you could probably even simulate trackball-like physics where you "throw" the pointer and then touch the pad to "brake" the pointer (like you would with a well-bearinged trackball) and trackballs ain't half bad for gaming. Not a mouse, but not bad.