I believe Simon is raising money just in case Westfield and other REITS start making bids to buy General Growth Properties. They want part of the "best malls in America" instead of the secondary quality properties they own.
Doubt General Growth will want to sell its best properties, but SPG may get some of the better properties in some regional markets. I have a feeling some private investment firm is about to buy General Growth outright, and SPG wants to be able to make a stock and cash counteroffer for the company should the situation arise.
Simon Properties is only partially dilluting itself by issuing additional 14 million in shares, but needs the capability to buy all or part of General Growth if the opportunity arises.