Is that a serious question, or are you offering up a snarky "yeah, and so what if they did, who cares" style argument?
The answer is: a lot
But really you need to possess the capacity to conceive of at least one example of some selfish desirable evil, before you can grasp the vast potential this potent advantage of pervasive eavesdropping offers.
The elementary starting point in all these eavesdropping shenanigans is to imagine someone cheating at a simple card game. If you're the dealer, and your opponent is betting real money on the outcome of the game, would it be fair for you to look at his hand, or worse, stack the deck in you favor before the game even starts?
But population control goes way beyond the elementary sneaking of e-mails and phone calls. Total population control would carry profound and harsh realities with it, like eugenics, human experimentation, blood sports and human spectacle, secret prisons, human trafficking, sex slaves, disappearances. Trivial things like stock market manipulation become absurd, meaningless anachronisms, as corruption is endemic to the scenario. Extrajudicial killings, summary executions, sovereign leaders can wave their fingers and have people put to death.
What's it like to be a slave? That's what the ass end of total poulation control looks like. It's like being an olympic athlete for Iraq, and then coming home to be tortured by Saddam junior when you lose.
For those that benefit from total population control, every decadence awaits.
But, perhaps your rhetorical question is something along the lines of the "human rights are inalienable, and it would be impossible for them to do special terrible things, beyond the terrible things we normally fear like chemical warfare and nuclear annihilation"
Actually, no. Under a regime of technologically enabled total population control, there would emerge a class structure so rigid and inescapable that essentially a very small number of people would be born into a lavish, but paranoid existence, and the rest of the world would suffer a permanent crushing misery, that starts with ordinary poverty and gets worse. All of it enforced by geosynchronous satellites and atomic clocks with nearly the precision of spinning neutron stars.
Keep in mind that this would be a new age of technologically enforced, drone enabled population control. A North Korean gulag is bad, and it probably doesn't get much worse than being an elderly uncle to the dear leader, and still meeting an end by kangaroo court trial and an execution where your ripped apart by dogs.
Unless of course, the dogs are robot dogs manufactured by Boston Dynamics (oh, wait I mean Google). But maybe they're just drones armed with ever more accurate hellfire missiles. And that pretty much sums up the blue sky possibilities, if we fail to preserve the old-world checks and balances that are quickly disintegrating in the face of advances in modern technology. Completely obedient robotic systems enabling a truly air tight subjugation of humantity beneath an ever watchful panopticon. And wealth disparities that look like an abyssal cliff separating hollywood visions of the future from the stone age, and sociopaths enforcing the existence of the cliff perhaps for sheer amusement, narcissism, or whatever irrational human foible (bodily fluids, mandrake) worms it's way to the surface of the human factors that control these things that now influence our lives.
How many drones can be controlled from a single data center? How many drones would we need to enforce a permanent world-wide police state, and surveillance apparatus?
Technology is a force multiplier, and it permits fewer and fewer people to achieve a great deal, and possibly much more, far beyond what we've considered "super human" in the past. But worse yet, it can mutate innocuous well-meaning intentions in perverse ways. Thermonuclear bombs and missiles have kept a lid on serious global warfare for more than 50 years, but only by virtue of two psychotic power structures promising to vaporize each other at a twitch. Is that "good"?
But let's say the worst happens, and an evil megalomaniacal cabal implements a true robot armageddon. Wouldn't such a terrible sociopathic technocratic regime just cut to the chase, and kill off every last human being that dissents? Probably yes. But only if artificial intelligence proves to be a stalwart companion, otherwise much of the general population of humanity would be permitted to live, if for no other reason than fear of lonliness and inbreeding, but undoubtedly humanity would still be subjugated at the whim of the technocracy.
Anyway, to sum up, part of the story would be a short violent transistion phase, followed by a miserable but infinitely stable, robot-reinforced tyrrany. Like a marvel team-up of Kim Jong Il and son along with Saddam, Uday and Qusay all chanting "O'doyle rules!" while eating hot dogs and drinking Big Gulps and watching robots rape political disidents to death in a stadium filled with their genetically engineered clones and sex slaves.
Dare to dream.