I do challenge the concept that all food sold is leftover. I have been at the huts several times where the crew had deliberately cooked snacks for sale to dayhikers. I have seen sheet pans hot out of the oven mid morning that obviously were not left over from breakfast. On the other hand I have also seen obvious left overs from breakfast/dinner also being sold. Given Alex's confirmation that the hut snack trade is a non taxable gratuity direct to the crew, its pretty intuitive to me that some crews are going to supplement their income with sale of intentionally cooked snacks unless they are specifically told not to. Hard to beat tax free cash. The crews by default are given a lot of independence to run the huts and I could speculate that unless specifically told not to do so that the crews are going to stretch the limits. Realistically AMC is better off ignoring the issue so they can have plausible deniability and crews as scapegoats if there is a complaint.
The question in my mind is if this is allowed by the AMC hut permit given the past rollback of hut sales required to obtain the new permit? I guess it goes back to the controversial hut relicensing back in 1998 where the USFS and AMC were both under external scrutiny. Reportedly prior re-permitting cycles were far less formal or controversial. I seriously doubt any forest service employee does routine audits if the huts are in compliance with permits unless there is complaint.