Good question. The house way says to split up five aces, playing two aces in the low hand. The exception is to retain the five of a kind in the high hand if you can play two kings in the low. My pai gow poker page indicates seven house ways from Las Vegas and Atlantic City, and all of them include this exception. The three from outside those two cities do not.
The probability of getting four aces, the joker, and two kings is 1 in 25,690,513. Assuming the dealer is banking, the only situation where this exception would help is if the player had a four of a kind or better in the high hand. The probability of that is about 1 in 300. The probability of both happening is about 1 in 7.6 billion.
According to the Nevada Gaming Control Board, there were 306 pai gow poker tables in Nevada in 2009. If we generously assume 60 hands per hour, two players per table, and every table is open 24/7, then it would take 23.7 years for this exception to occur and make a difference in the outcome.
So the casinos are requiring every single pai gow poker dealer to memorize this exception when it may never have made a difference in the outcome in the history of the game. I speculate the same person who thought of adding that rule was the same person who decided that the A2345 straight, known as "the wheel," is the second highest straight.