This is because of the way that TP adds it's copyright to the bottom of the page. SMF uses what's called an output buffer. It holds all the page code in a buffer until execution is done, then the buffer is output to the page. TinyPortal (and many other big mods now) add their copyright by searching through the buffer for a certain text. In TP's case it looks for the text "Simple Machines</a>". Once it finds that text, it appends its own copyright to that text. You can see how this works by looking at the tp_addcopy() function at the bottom of the TPsubs.php file.
So that also means any time you create a link with the exact words "Simple Machines</a>", TP will append it's copyright anywhere it finds it on the page.
The fix will be that the code will need to be changed to look for something more specific instead of the generic link text.