This is about the four horsemen -- Amazon, Apple, Facebook, Google -- and how Amazon is moving forward with Facebook and Google, and perhaps into the lead.
The true purpose of Google+ is to accumulate more information about you, not to be a "social network" per se. The latter is the means to the former.
Amazon has a huge amount of exceptionally high quality ecommerce information. They have "only" 200 million users, but info about what they actually like -- because they buy it -- not what they say they like. Plus, for search -- if you're searching for products/pricing, you might already use Amazon not Google.
Login helps Amazon extend information beyond ecommerce -- and potentially pull clearly in the lead in that regard compared to Facebook and Google.
Apple...has a problem in this regard. Excellent company in many respects, but falling out of the pack in this regard.
p.s. In all of the above I'm only talking about it from the company perspective. If you think it's not necessarily a good thing for a company to have even more complete data about you, I wouldn't argue with you.