Although Starwood has historically played with the right of a non-SVN unit owner to deposit a non-SVN week, they needed help to pull off this new caper -- and I think the real culprit is I.I. Why is that?
First, Starwood has had a very cozy relationship with I.I.; think about the retroactive reassignment of units bulk banked by Starwood over the years.
Next, there is absolutely nothing preventing I.I. from accepting non-SVN reservations; other exchange companies do.
Finally, as rickandcindy point out above, this is new arrangement may not be in I.I.'s interest; they will end up with only "average" deposits rather than the mix of the previous hand-selected deposits by knowledgeable owners and the previous bulk deposits by Starwood.
My theory: I.I. is running scared. Disney just moved to RCI. Marriott is (maybe) moving to an internal trading system. Starwood already has a relationship with RCI on its older resorts which could presumably be expanded. If Starwood also switched its primary external trading company, I.I. would lose much of its remaining high-end exchange resorts, and losing Disney, Marriott, and Starwood within a year or two would be devasting.
So -- and this is total speculation -- Starwood has enough bargaining power with I.I. at this particular point in time that it can force I.I. not to accept "unauthorized" deposits, even if the owner has the legal right to use, rent, or deposit a reserved use week. What Starwood previously tried to accomplish by misinformation to the owners of the units ("we will deposit a week for you"), it will accomplish by forcing a change in its relationship with I.I. instead.