That chicken didn't have the entire head cut off. The 'executioner' missed the brainstem, which controls basic cardiopulmonary functions and movement. It would have died within a day or two if some weirdo hadn't kept the wound from getting infected and tube-fed it grain
This. A complete decapitation (which is what happened when I butchered chickens) would not result in a bird continuing to "live".
Why keep a snake? Why keep any animal? Because you enjoy the animal, find something beautiful and fascinating about it, and it fits seamlessly into your lifestyle.