"Technically", if the files in question are not being redistributed, the link can go if it's GPL. Letting someone use it on your site isn't distribution. :-\
I'll be the first to admit though, that any theme that has spam links in the footer gets any and all links removed. It's a disgusting habit that has been gaining momentum lately, and I hate it. Mainly because a lot of the end users of that theme know nothing more than to upload and activate. It's no different than link farming, just in another disguise.
Of course not all theme authors are slutty spammers. Those I have no problem keeping, unless I go through and hack the theme to death to make it validate, display correctly, etc. But it's more than one line edits I'm referring to. I'm sure we all know the couple of themes out there that need a ton of work. :-|
Now, if the links are removed, and someone says they made the theme, it's definitely a violation.