Because isn't this diluting the brand, somewhat? It's prominent, at the top of the page on the desktop site and the storymakers part of the logo takes up more space than the mumsnet logo.
I appreciate that sponsors and advertisers keep the site going, but this makes me question the impartiality of Mumsnet. However good the storymakers site might be (I don't know, its irrelevant to my kids,so I haven't checked it) it belongs to a manufacturer of cheap chocolate, marketed towards children.
Is that really an association you want to make so prominently?