I have noticed that once a planet is bombed and taken over; the previous owner's structures remain. Me and an AI created a rather strong chokepoint as follows;
I'd bombed and taken his planet, but left his turrets. He offered a ceasefire while I built my own turrets/tactical structires. Our alliance grew stronger and I maxed out my tactical structures while, as far as I could see, his structures were still there and working fine too; all in all giving us extra tactical points worth of structures in orbit around the planet.
If a particularly strong alliance is formed in multiplayer, it wouldn't be too hard for one player to max up his turrets/hangars, break the alliance and let his planet be bombed, re-ally and then let his ally max out his turrets too; giving the potential for 70 gauss turrets on a Terran planet.
It must be noted that this is only as strong as the alliance... but still is this a bug/exploit? If so, possible workarounds would be for to have any (not just your) structures count towards the respective caps, or for structures not owned by the planet owner to slowly decay.