I believe it is your beloved big brother watching out for your safety. Leaving an un-sprung chuck key in the chuck and turning on the drill press can turn that key into a dangerous missile destined to poke out an eye, or worse. The spring keeps you from leaving the chuck key in place thereby removing the threat of losing any eye while simultaneously facilitating the pinching of fingers and the learning of new words with which to describe unfortunate shop experiences.
“We all die. The goal isn't to live forever; the goal is to create something that will.” - Chuck Palahniuk