It isn't on the new premium themes, though, which seems odd. Good, but not exactly in keeping with the stated policy.
In spite of the anti-option policy, you do need to provide an opt-out for premium theme users. Some premium theme users are going to love infinite scroll. Some are not. Most of them will have invested in their theme on the understanding that it is going to continue to look and function as it did when they first bought it, and messing around with it without their prior knowledge or permission is not going to play well. They paid for their theme; the least you can do is pay them the courtesy of chosing whether they want this new feature rather than forcing it upon them. That, or add footer widgets to any theme which doesn't already have them. (Added bonus: you get to claim this as another theme improvement.)