If you're being spawn camped, you can't exactly return fire and spawn camp the enemy with your own turrets. Hence why its called "SPAWN-KILLING", you die at the spawn, you don't get to return fire, advance, or even move far enough to evade, you die.
I may be gone from the game, but I've played enough video games to know that when your team is getting spawn killed, you can't do much.
(Be VERY thankful they haven't added mines yet, believe me, spawn mines are the worst. You die instantly before you can even move your cursor to aim.)