If the theme is free, I feel that you have to keep the links intact. As much as I hate it, I leave the links on all of the themes that I'm using for free. I do use the
TAC plugin to check for malicious or hidden code though.
I wish more free themes came with the option to pay a nominal fee for removal of footer links. I've passed on some great themes because there were links out to sketchy sites or some such.