I just now realized that the prompt in the upper right sign-in block raises some metaphysical questions. The prompt is not for "Registered User" or "Approved Customer." It asks if you are an Existing User. Is it not metaphysically impossible to be a non-existing user?
And would it not be a logical contradiction to attempt to be an existing non-user if you were using the site at this moment, regardless of your contractual status? For one thing, you can view without signing in. I do that, sometimes, just to see what is happening. And the site software does allow you to log in anonymously, does it not? And if you just view the homepage and click off, you have still used, and are therefore an existing user (of low engagement).