So let me get this straight. If they ship me something that I did not order, I can either:
a) keep it at full price
b) say that I don't want it
In the second case, they will either eat the shipping cost both ways, offer me a discount, or just give it to me for free.
Why would I ever select option a? Yes, then they wouldn't ship me stuff ahead of me asking for it, but considering that I typically don't even know what I want until I go looking for it, I'm not sure how Amazon is going to do any better. Especially based on what I see recommended (maybe 1 of 10 items I *might* want)... seems like it's conceptually doomed.
I could see this being useful for businesses who need JIT delivery, but there's already a system for that.