I believe all of the US has wildlife regulations that are "permissive" as
@Doug3006 points out. One of the clarifying provisions in the actual laws behind the regulations will specify that if it isn't listed as an "allowed" activity/quarry, it is illegal. For instance, I have not found a list of every bird in any state that is protected/illegal to shoot, only the (short) list of what is legal to shoot, and under what conditions (season, weapon, etc.).
Colorado law, the CRS (Colorado Revised Statutes), sets up those rules we see in the hunting regulations. The regs (and laws) make it very clear that if it isn't specifically allowed, by definition, it is illegal.
I would highly recommend you don't shoot the stray moose in KS or OK (or any state that doesn't have resident moose) that do occasionally wander through. Even though there is nothing in the hunting regs concerning moose, the law behind those regs (that very few people ever read) is clear that the lost moose is, in fact, illegal to shoot.
To avoid any question of legality, and to avoid the problem other states have with feral hogs, Colorado added to the law that any hog not confined/fenced in or otherwise controlled, IS fair game. I don't even think you need a hunting license IIRC. Colorado does not want feral hogs, lol. They didn't add that paragraph into the regs for a couple years after the law was passed.
I should also point out, there are some states I'm aware of, like TX, that do make it legal to shoot non-native, non-regulated, large animals. Year round even. But that is also stated in the laws that set up the regulations the DOW puts out.
Just realized the thread this is in. Talk about thread drift, lol