It has been seen in later sieges that players of a side can hop on movecraft of their opponents and apart from sitting on a periscope to entity push the pilot thus invalidating /dc movement and obscuring view, they are able to shift clock aim the crafts cannon aswell making a craft technically useless if not harmful to the original pilot's side. I dont really need to state why this should be illegal if dragging is but I have seen this occur in several sieges much to the displeasement of movecrafters and when said move failed from the opposing side they'd call 'drag' which is silly. But I think we can all agree if dragging pvpers with movecraft is illegal it should also be that pvpers cant obscure movecraft view and dc movement with entity push and also cant redirect and fire the crafts cannons.
Best way I can see how making people of an opposing side unable to fire your crafts cannon is perhaps a plugin restriction as for the rest it would have to be proof provided the same way as dragging.