Also worth noting that having it so that fighters being unable to be directly fired upon when stationed on a planet (since, as per game mechanics, ships under 500 tons disregard the gravity constraints of other trans-Newtonian craft and can land on system bodies) has an extra benefit: it makes it so that you can't bypass an enemy system as easily by simply neutralizing its present navy. What I mean by this is that, unless I'm mistaken, presently with how the game works if you destroy all enemy crafts in a system that you can then proceed onward to other systems without dealing with any colonies of theirs still present.
However, if you can't directly fire on fighters present on a planet, then it's possible that any colony could have potentially innumerable fighters (and build even more) that they can deploy against any enemy forces and then retreat back to the host planet where they have both the benefit of STO fire covering their retreat, and not being able to be targeted effectively at a range by guns that outrange the STO weapons.
In other words, it makes it so that a sufficiently-reinforced colony with plenty of fighters and STOs can force a threat to need to mount a full-scale planetary invasion of that colony (or otherwise destroy it) strictly in order to continue their broader invasion of an empire unimpeded. That is to say, colonies gain a greater capacity for system-wide deterrence in the event of a full-scale war rather than simply being able to defend themselves.
