This macworld article has the actual quote:

http://www.macworld.com/article/54948/2007/01/80211nfee.html

“The nominal distribution fee for the 802.11n software is required in order for Apple to comply with generally accepted accounting principles for revenue recognition, which generally require that we charge for significant feature enhancements, such as 802.11n, when added to previously purchased products,” said Teresa Brewer, Apple’s Mac hardware public relations manager.