Apple's gesture handling API lives in a completely different environment.
But I generally agree with you. It's up to client/toolkit to handle complex gestures. However, even Apple abstracts pointer movement and scrolling on input level, and that abstraction includes inertial scrolling. Rubber band effect doesn't really interwine input with UI logic (it can easily be entirely encapsulated in the widget itself) and for other gestures you're going to use some gesture-specific API anyway. The scrolling inertia is a function of the input, so the proper place to calculate it is on the OS side, not in the client/toolkit - it can be in toolkit only when the toolkit itself is a part of the OS. You don't have such toolkit in the GNU/Linux land.
But I generally agree with you. It's up to client/toolkit to handle complex gestures. However, even Apple abstracts pointer movement and scrolling on input level, and that abstraction includes inertial scrolling. Rubber band effect doesn't really interwine input with UI logic (it can easily be entirely encapsulated in the widget itself) and for other gestures you're going to use some gesture-specific API anyway. The scrolling inertia is a function of the input, so the proper place to calculate it is on the OS side, not in the client/toolkit - it can be in toolkit only when the toolkit itself is a part of the OS. You don't have such toolkit in the GNU/Linux land.