...and subscribers get to have links in their signatures, too.
However, a signature doesn't change if your subscription lapses, so that's one reason you might see non-subscribers with longer sigs and/or sigs with links. Another (increasingly less common) is that such sigs were grandfathered in when the current limitations were instituted oh-so-many-years ago.
I used to have a big signature, but then I took an arrow in the knee...
Yeah, mooks like us get 1 line, unless you still have a big one from way back when, which I did until a few months back when I instituted my current beg-for-xp drive.