More textscreens on the server, more props, more lag. When it gets to a severe situation then maybe we'll waste another half FPS on a textscreen up there. Until then it's just a regular server rule.
As it stands, during DEFCON 6 or any ordinary situation, you may not be on top of ATC. You are to be at ease, doing your regular duty and that in no way would require you to be on top of ATC (unless you make a meme excuse to try and play against the rules). When outside of such DEFCON and either during a combat situation or with a good reason, you may get up there for strategical purposes such as getting a good line of sight with a sniper because that is utilizing your envinorment - but also realize you are making the ATC a more focused target if enemies become aware that's where they are being attacked from.
So, just like cloaking, saber ignition, firing, etc, is not allowed on DEFCON 6 neither is being on top of ATC. Once you're in a combat situation, then it's strategical advantage.