Uh... because... why?
The SE costs double what I'm paying for my CE, comes with all of the Stealths instead of only certain ones (but I'm thinking they'll be available as DLC later a la PSP anyways), a wallet, and what's essentially a lunchbox. None of that strikes me as worth an extra $100. Oh, and then the shipping and likely duty charges, so well over $100. (EDIT) Woops, forgot, a USB stick! Such value!
And the SLS prize would be out of my reach anyways since I'm not living in any of the countries taking part in the competition.
Another drawback would be having to create a European PSN account if I want any future updates or DLC. That doesn't affect me much, but I imagine for people with a lot of stuff tied to their current accounts might take issue.
It's a brilliant marketing move though; that SLS prize is going to be easily paid for by people eager to buy these Signature Editions. And probably provide PD and/or Mercedes with some sort of extra profit as well.