Why put your logo on anything? Are you saying once someone has bought a Nike shirt they don't need the logo because they have already made the purchase? Sure websites are a little different, but just think of all the people you could drive to your site who simply walk by someone on this site, are curious, and then go there on their browser.
There is a reason every major company has their logo on their website, and they can make the same arguement, 'well the visitor already found my site, who needs a logo' . Its branding.