It's a mix of a few things.
One of these things is to attempt to eat into the casual market and slow down the Wii.
One is to make Sony a lot of money for each Move sold. I imagine they profit quite a bit on each controller sold. Same goes for PS Eye and the Nav-Con.
One of these is to simply give 3rd parties another excuse to develop on PS3. I mean... Look at it this way. Why make a game a Wii exclusive, if you can make it a Wii/PS3 Multiplat? Anything the Wii can do, PS3 can also now do. Providing it's not fitness... Of course. We are already seeing possiblities with Epic Mickey PS3 (and 360) porting discussions happening.
Another reason is to get Sony used to developing for a Motion controller for the next generation.
