They aren't turning back though. This is just an example of the stormcast fighting alongside their allies in the grand alliance - just like that time when they fought against Nurgle with Sylvaneth. An impermanent grouping of forces at best, but it is something Sigmar MUST do to keep his allies happy. An ally that don't help you out is no ally at all - same goes for the corsairs, they either got some favours, or gold out of this, or got strongarmed into the fight.