Its all about choice and revenue...
If an end user could buy an AMD cpu + chipset then run SLI AMD would still get money for the chipset. With the current situation anyone that wants to have SLI is forced to buy an Nvidia chipset on an AMD platform.
Choice is always good as an end user (competition invariably leads to lower prices), and it would be good for AMD because they could increase their share of the chipset market on their own platform. It would however be bad for Nvidia since anything AMD gains Nvidia invariably loses.
Some people are always going to want to use SLI, regardless of whether crossfire is better or not. If AMD could sell chipsets to that market as well they would make more money.
But hey, since Nvidia would probably rather release a FX5200+ than give AMD SLI its a bit of a moot point