On the bottom of this page is a "Powered by vBulletin®" link.
WordPress has similar, and many free themes also contain links back to the theme author's site.
IMHO unless you paid for the theme, a link back is small price to pay. In fact, I've seen themes that "require" you to keep it there as part of some sort of "use agreement" though I doubt such is very enforceable.
If I'm correct, shafiqul1976 is wanting to remove/replace this.