Legal. This rule mainly applies to the marine commander, as he can instantly, anywhere on the map, with absolutely no risk at all, and with only a very modest loss of resources (assuming he recycles) drop a structure that has a whopping 5000 hitpoints, which would take even an Onos a while to chew through. An unbuilt OC has only a tenth of that (or does it? Unbuilt structures have 50% of their normal hitpoints, right?), and the gorge has to place itself in harm's way. You don't have a chance to kill the commander if he places a structure near you, but you can hunt down and slaughter the poor gorgie if he tries to infest your turf.