This also gives the Hessians a breather. You can't log in and fly 10 feet without someone trying to kick your ass as a Hessian. Either RM is on you (if you stray from Dresden), Bounty Hunters are on you just about any place, Unioners are on you pretty much anywhere in Rheinland, and Corsairs roam the nearby omegas.
Making the Unioners more friendly with their nearby unlawfuls makes sense and it takes a little pressure off of RHA. How exactly does it hurt the Corsair cause? Were the Unioners joining Corsair fleets to go engage their enemies? They seem like they've stuck to Rheinland only as they should and their departure from Corsair alliance reinforces their relationships (positive and negative) in their home areas. Seems like a good move overall.
"To gain a hundred victories in a hundred battles is not the highest excellence;
to subjugate the enemy's army without doing battle is the highest of excellence."