You can benefit from the lock in. Amazon pays a fortune on my shipping for Amazon Prime. I ordered about 90 times last year, and got 2 day shipping on all of them. It replaces going to the store for many things, so more important to me is the time I saved shopping.
The upside for them is I spent about $4000 on Amazon last year, which is way more than I would otherwise. I don't know if it is good for them, but it sure seems good for me.