I imagine mental stimulation is what's at stake, and I've seen enough videos of snakes being excited over already dead rodents. Maybe something similar to depression is occuring in snakes otherwise bored with their overall keeping, but given that some snakes eat dead things in the wild already, I doubt that has much to do with it. Especially considering that if they deem the meal good enough to eat, they're probably fairly happy with it. Just give them nice alternative views now and again, along with exercise. Keep the cool blood flowing. My views are all formed from second-hand experience, though.