Why does American satire always assume that the person viewing it has the education of a 7 year old, and that they need everything labelling and reams of text to explain the point in simple terms?
Partially it's tradition:
Thomas Nast is really the father of modern American political cartoons as distinct from their British counterpart, and he always continued to label everything.
I would suspect that there's also a functional element in that most of these cartoons are national, and thus distributed to people that might not be as familiar with the situation in particular. For instance if Walker from Wisconsin were referenced in conjunction with a broader issue, he'd probably need to be wearing a banner that said "Walker" as someone in Arizona probably wouldn't recognize a caricature of another state's governor on site.
If a British paper made a caricature of Thomas de Maizière stopping Gérard Longuet from imposing a no-flight zone over Libya, there would need to be some kind of label as most British people wouldn't know either people on sight - even though they live and operate in large national capitals much closer to Britain than Arizona is to Wisconsin.
Alis Volat Propriis; Tiocfaidh ár lá; Proletarier Aller Länder, Vereinigt Euch!