Social sign-ons have been shown to reduce friction when getting visitors to register for a subscription or membership site. And the benefit is that social platforms, like Facebook, LinkedIn and Twitter, don’t cut into your profits by offering such a service.But there’s a disruption brewing. Amazon just announced plans to make it’s “Login with Amazon” feature available to all sites. The technology is already being employed by Zappos and Woot (both of which are already owned by Amazon). Amazon is using the lift in conversions from visitor to registered user as a marketing tactic to lure more users.But is this new technology in publishers’ best interest? On the plus side, it can reduce registration resistance. And users can take advantage of Amazon’s Web Services and other technology bells and whistles.But on the *very* negative side, it doesn’t look like publishers will get access to credit card numbers. You will get access to a name, email address and zip code, which is better than some social networking platforms. But by controlling credit card numbers, Amazon is basically creating a closed environment like Apple, where it can start to dictate terms with more impunity.For small, mom-and-pop operations who want to avoid payment processing problems, Login with Amazon may be a boon. But publishers worth their salt should think twice about installing it — and perhaps avoid it all together. Because as we’ve learned from our original sin online, it’s hard to squeeze the toothpaste back into the tube.