I don't understand why you won't accept that, unless you want to act as you created the site, which is wrong.
Alot of companies request this -- think, if anything, about print design. I can NEVER put my name on a professional brochure, catalog, or marketing layout -- same thing as a website, only there's printing. Some companies will go so far to make you sign a waiver dis-allowing you to put it in your portfolio at all (or even stating that you worked on the site, which is the case here I believe). So think of the reason for the trade off or the benefit and do it or don't. Some of the benefit is just personal practice, helping out fellow peeps that you know are good workers -- and if he's going to link to your personal site, then with his hit count that might be worth it for your name alone.
Remember, the Nike swoosh was designed by University of Oregon student Carolyn Davidson in 1964. She was paid $35 dollars for the design, and received nothing more. Welcome to corporate America, it sucks, but it's our industry.