I think the reason Imu doesn't go sicko mode with their nuke is the whole thing with the balance of the world. If there are no pirates then citizens have nothing to fear so no benefits for the WG.
And in this scenario it seems that the reason they're nuking this kingdom right now is the the royalty were egotistical and sat or touched the empty throne. Since the royals were warned no to during the reviere with Stelly even being tempted.