My view is basically that there is a price for security. If you want to put up defenses, you have to pay the upkeep on those defenses. However, being able to power down those defenses when you absolutely need the power, without having to scrap them to get your economy functioning again, is also important.
That leaves a "remainder case," which is basically the one that bugs you: that you can power it down when there's no enemy but yet when you want the energy back. Doing that in an automated fashion would be unbelievably complex for the player, because they wouldn't ever be able to energy budget.
And more to the point, it's not really intended that you be able to save that energy while still having the turrets for normal defensive purposes; that's kind of skewing the balance a bit. And making these specific units mysteriously not pauseable would be annoying for other reasons, particularly due to their higher energy costs.
Which leads us back to the remainder. Which I believe is the least of all the available evils. I've been around and around on that remainder for the last two and a half years, and I haven't ever found anything that didn't have worse problems. There might be a solution out there, but my hope for that is wearing thin, so to speak.