The only time that it is legal for a government to control somebody's speech is when they're actively calling for harm. Something like screaming fire in a movie theater can be prosecuted. Threatening violence, calling for violence, or creating terrorist organizations are all illegal and are regulated by the US Government
Zoos and wildlife sanctuaries keep animals safe and preserve endangered species. Plus, they model their captive habitats to match their natural ones. They don't have to hunt or worry about dying!