 Why shouldn't Purnie stop time?
When Purnie had stopped time, the bees—like all the other creatures he met—had been arrested in their native activities, and he knew that as soon as he resumed time, everything would pick up where it had left off. He chose to ignore the negative maxim that "small children who stop time without an adult being present, may not live to regret it." It was time itself that Purnie had stopped, not the world around him. The tripon thought Purnie's feat was superb. Purnie ran from pillar to post, trying to see and do everything at once. The animals stopped so suddenly that Purnie nearly tangled himself in their heels. Purnie shrieked with joy at this new frolic and promptly stood on his head. When they stopped, Purnie stopped.
 Stopping time consumes massive amounts of energy.
 Small children who stop time, may not live to regret it.  Purnie may be abducted if the animals know he can stop time.  Purnie may not be able to get time going again.  Stopping time consumes massive amounts of energy.