I can see both sides. When a hawk is done, there isn't going to be a whole lot that is left over, let alone go to waste. There are plenty of scavengers, (at least around here) that need to eat too. Opossum, two different species of vulture, coyotes, crows, etc. So I doubt that much is really being "wasted" per se.
I am only guessing here, but I would venture to say that the reason the state has said laws in place is because if they didn't, you would have people out "hunting for rabbits" when really they are duck hunting, and taking 2-3 ducks in their game bag out of season. "Sorry Warden...we were going for rabbits, but she just kept going for these ducks in that ditch." Whereas, if you made it illegal to take said out-of-season game into possession, you have a lower likelihood of people actively going for out of season game since there really isn't much benefit to the hunter since he/she can't keep anything.
Also, the law USED to be that if the hawk took out-of-season game or a protected species, you had to transfer it off of the game and leave the whole carcass. The state realized that most likely led to more unnecessary waste, coupled with the fact that hawks hunt anything, and as such, modified the laws to allow the raptor to eat what it kills. So in a way, the state has taken measurements to address your concern, while still attempting to keep the hunter/falconer from actively going for game that he/she shouldn't be after.