It's the same marketing/pricing scheme that ImagePrint uses for their RIP: The more expensive your printer, the more expensive the RIP is, for essentially the same software.
I suppose the justification in each case is that it costs them more to research the more expensive printers/camera bodies/lenses, and folks like us who spend more money on our equipment are likely to be willing to spend more on their software too. I certainly hesitated before I bought DxO Elite 4, but I have found it well worth the big bucks.
And Alain's excellent guided tour of version 5 redeems that one in my estimation.