One definition of "incidental" is that something is liable to happen as a consequence of an activity. The intent of the activity is not to trip or foul the defender, but that is indeed what happens.

So the question is... is it legal for the shooter to kick his feet out like he did?

Maybe, but here we would tend to use the rules book definition so as not to cause confusion.

You're right.

The defender jumping into that space can be considered a normal defensive movement. If the defender jumped into space that the shooter had legally occupied, then a foul on the shooter can not be called. But that's not the case. Therefore I see that the contact was caused because of the shooter kicking his feet out into that space, and such contact tripped the defender.

The fact the defender could have gotten injured should not be a factor... that is right. But the trip is an illegal act, and penalizing it can help prevent something like this happening again in the future. And a possible future trip may not end as well. Hence... preventing injury.