It's in the rules. It was put in there about 6 years ago to prevent guys from pulling out of doubles early claiming injury to make sure that they make it to the next tournament in time. It was more intended for those that are playing qualifying of the next tournament, but in order to cut down on the gray area, they did it across the board. If you pull out prior to or during a doubles match, you get prize money for the previous round.
In 2004 (I think), I was chairing a doubles final in Champaign. Gimelstob wanted to retire in the second set with an injury. They were down a set and at least a break. He said he wanted to retire. The players had shaken hands. I called the supervisor to let him know of the retirement. He came to the court and told them of the rule. Gimelstob felt so badly for his partner because he just screwed him out of a decent amount of money. Gimelstob begged the supervisor to let them finish the match. I think it was only 2 games. The supervisor reluctantly said ok. Gimelstob and his partner went back on the court and tanked the next 2 games and the other team still won. Gimelstob was legitimately injured and had he known that rule, he would have continued without saying anything.