What's so wrong with being a poor sport? Basketball is LeBron's job. He failed in his goal of leading his team to a championship. The loss will probably directly impact LeBron's bottom line in lost additional endorsement deals and prestiege. That's the real-world equivalent of failing to close a deal or losing a huge sale to a competitor. We don't expect Boeing to call Airbus and congratulate them on scoring a government contract when that loss will cost Boeing billions.
Even if you limit the conversation to individuals, the argument falls flat. I highly doubt that Christian Bale called Ben Stiller to congratulate him on "Night at the Museum" beating "Terminator 4" at the box office. Why should we hold LeBron to a different standard?