A key here is that with "normal" customs on arrival, you can bring snacks, including fruits, to eat on the plane and discard legally before reaching immigration/customs. There are presumably approved methods for disposing of the trash from inbound planes and the cans in the arrival area to prevent spreading pests. But with pre-clearance, you will enter at a regular domestic gate, with no special trash disposal way to ensure you dumped anything inadmissible anyway. So you inherently can't bring such products on the plane (but can purchase them after pre-clearance if available, with no need to further declare); the customs agent was correct to prohibit the cut-up apples, and as others have said this is nothing new.