It is legal in Colorado to hunt with the .50 BMG for big game as their is no restriction on the cartridge. The rifles are just too big and heavy to be a popular choice for big game. I know I sure wouldn't want to lug one around very far above 7000 ft.
However I do think Idaho was trying to ban the use of this cartridge for big game hunting. They couldn't ban the caliber because of Muzzle Loaders, so they banned rifles weighing more than 16lbs IIRC. They were more concerned with where the bullet would end up than the animal suffering. Over pentration is an issue when using a cartridge this large.
Cornbush beat me to it!