Precisely as the others have said, this place costs a fair bit to run and activity is ever growing and with the monthly goal not being too good for the last few we decided to look at alternative yet beneficial to the members' means of generating revenue. Affiliating with good efficient and well respected websites was the best option. If you note the ad itself is no way shouting at you or in the way, it's got a small image of the game in question and small text for the link(s).
And in true TDUC style we don't force you to view it all the time like other places, you can simply click the red X and it will go away. Also like the message says itself we want you to suggest to us good places where we can affiliate with, we're not only going after the money, far from it in fact because the place we are currently affiliated with is low paying compared to some but their service and reputation is very high and that's worth more to us than a high paying corporate machine that pays out well despite not providing the best of service.
It's also not everywhere, it's only up for certain games and big releases. When Atari/DP sort themselves out I'll even see about hooking up with them in some way if possible.