Sure, if we follow a link to a site’s home page, we see both “Sign Up”
and “Log In” buttons. The former leads us to “Welcome! Let’s create your
account.” So far so good.
But if say we followed a link to this comment that you are reading, then
there is no “Sign Up” button, only a “Log In” button, with the
aforementioned over-assumption that the user has been there before.
But wait, if we scroll up some more, then suddenly…
Sign Up appears.
OK, now let’s dive into even deeper Mr. Nebbercracker thought here.
Even if “Sign Up” was present, by the time the user scrolls down (and reads the e.g., nonsense (that e.g., I am) posting), and gets the urge to reply, by that time the SIgn Up button has disappeared and all that is left is the Log In button with its unfortunate assumption that the user is “back”.