The Maw-Walkers have been Zovaal's largest adversaries, and half of them belong to the Alliance. Not to mention Bolvar and Jaina too. The rest of the Shadowlands denizens have proven almost entirely inept. So distracting even half of us gives his forces a whole lot of freedom. Also, attacking Azeroth doesn't just have to be for Stormwind -- that can just be one of the primary objectives. If the Mawsworn launched an invasion from the rift in Icecrown, that's a whole lot of carnage to leave even if they just bee-line for Stormwind.
Uhm no, Anduin attacking Stormwind wouldn't make either of them bad. It'd just be an interesting development and cause some tragedy, you know, since its war?edgyness and "PARRELELL TO ARTHAS" They just want Anduin to be bad or the Alliance bad.