If you designed the ad you'd put it in text/editorial and your goal will be to get the visitor to click on it. You have no incentive to promote their site/product to those visitors who are NOT going to click on the ad.
However, the way they see it is that they'd like every visitor to see the ad AND THEIR LOGO whether such visitor is likely to click ... or not. That way they get some sales which they pay you a commission for and they get a lot of FREE "branding" assistance for all your visitors who keep seeing their logo but which they never pay you for ;-)
(But that's the nature of the CPC advertising beast)