WTF is with that. Why would anyone in their right mind go to the bother of putting a link to a page when you're ALREADY ON THAT FREAKING PAGE?
as a design type guy, I can field this question.
It is common practice to have the banner image on sub-pages be a link that brings you back to the home page of a given site.
for example, on the page:
http://www.microsoft.com/downloads/search.aspx?displaylang=enthere is a microsoft logo in the top left hand corner. if you click on it, it takes you to the microsoft.com homepage.
This is kind of dumb if you are ALREADY on the homepage, however, more then likely, the site is designed as somewhat of a template. The header, footer, menu, style and other elements that are common to all pages are seperate from the actual content of each page. That way, if you need to make a design change (or update the menu or header image) you only have to change it once, on the template, instead of updating every single page of the site.
I suspect that is the case here, and the header is designed as a link because that same code is used for all pages of the site. most of which should include a link back to the homepage.
EDIT: this still a bit of lazyness on the part of the programmer as even a template site could be programmed to not include the link on the template when the content is the homepage. on that microsoft page above, once you click to the homepage, the logo is no longer a link.