I find it quite weird. Why do they think they have to advertise their services there, especially since the audience is webmasters? It's not like AdSense is some completely new product that no one has ever heard of before.
I see your point but on the other hand who else would they advertise it to if they don't target webmasters?
Webmasters of course use AdWords a lot to get traffic. They are more aware of the program than "product" sellers. But i admit that they have better options to advertise. weird yeah!