Why do we need ads in RSS feeds?

Already recognized that there appear more and more ads in your RSS feeds?
Right now I am lucky to have just a handful feeds with those annoying ads in my Bloglines account, but I am pretty sure that there will be more and more in the next weeks and months, because Google has announced the official beta state two days ago.
As I read in Moe's blog the RSS-Blogger wrote down a nice ranking (link to a german article) for feeds, I totally agree with. It's worse enough that you have often only previews of the articles and not the complete blog entry. But ads in it make it not better. And I don't get it, why I have to visit the website to read the whole article. I don't follow ads there either.

Technorati Tags: feeds, google ads, rss