Because there isn't a moving ship to shadow the deepstriker if the opposing player's ship disengages.
The whole logic behind the ISC now being able to disengage in a Planet or base hex is because of the Sensors and detection network.
I'm sorry, we don't need to make things THIS complicated.
Well wasn't referring to this server setup in the first place.
In the second place, if you win the mission even near a planet you are allowed to fight your way back, the planet is still there and its sensors exactly the same as if a live player was remaining, yet you don't have to self destruct your ship after winning the mission due to being hunted down by some fleet. The only difference with a disengement forced self destruction is that a non stationary opponent remains, thus the logic would actually be that this live opponent (perhaps bolstered by the planet/base sensors) is able to track your progress.
As far as complicated, just look at the build points/OOB threads