I think the simple answer is that the 950 is two generations old and has been discontinued since early 2005. They have to draw a line somewhere, and since they rolled that offer out last fall (when they wanted to give people who were shopping then an incentive to invest in either platform in spite of news that a successor was already on the drawing board) it made sense to tie the offer to those two products. The 950 had already been discontinued for nearly two and a half years when they announced the deal.

I'll offer a loose parallel, but be warned that it is very loose. if I have a copy of AutoCAD 2005, I am going to pay a lot more to upgrade to AutoCAD 2009 than someone who is upgrading a copy of AutoCAD 2007. Actually, in that example, I'm going to pay full list price for a new license, since they quit offering upgrades for versions that are more than a few years old ("retired" in the Autodesk lingo) - if I'm lucky, I might get a discount, but it's not going to compare to the upgrade price.

Here's a somewhat less tangential question: if you extend the offer to 950 buyers, what about the 1050 buyers who were around back as far as 2000?