In Major League Baseball why is an automatic triple awarded the batter?

Jokes used in movies and cartoons notwithstanding, throwing the glove to try to "catch" or slow down a batted ball is forbidden by the rules. When the umpire calls it, the batter is awarded an automatic triple (meaning that all runners ahead of him are allowed to score freely) and it is also a live ball, so the batter-runner has the option of trying for home if possible. Similarly, it is against the rules to take off one's cap to use it as an alternate "glove".