Again, nobody is saying nothing should be done to stop terrorists like the San Bernardino shooters. What they are saying is allowing the government to violate rights just because it feels like it will inevitably lead to it violating the rights of it's citizens. Permitting such violations in one area will also open the way to violations in other ones, as astute commentators observe:
“‘We have no sympathy for terrorists,' wrote Cook in his open letter to customers. Surely it’s a sentiment we all share, but Cook has empathy for his customers and knows the risks they'll face from even a single act of compliance on Apple's part. Cook made it clear that Apple cooperates and assists with requests wherever possible, but they have never given access to end-to-end encrypted data, because they can’t technically do so and they have fought every request for digital backdoors into secure hardware and software.
Cook and Apple are, in some ways, just like the rest of us.
They want the terrorists stopped, just not at the expense of every shred of privacy iPhone customers enjoy. But what happens if Apple loses? The FBI gets what it wants, accesses the iPhone 5c, pores over the data — and then what?
What we face is a bizarre risk of success. Who knows what the FBI could learn from access to that phone. Will it trace a thread directly from the San Bernardino terrorists to an ISIS cell in Syria? Will it provide phone numbers, locations and logistical strategies? It’s that kind of jackpot information that will generate an existential crisis for privacy and security proponents.
How do we deny law enforcement when there is so much potentially attainable and actionable information?
If Apple is forced to write that software and the FBI finds nothing or tries to build scant evidence up into a big win, everyone will know. We will still be on a slippery slope, but I bet many tech companies will be able to point to this moment as a law enforcement overreach that netted virtually nothing.
Success, though, will turn that slippery slope into a landslide. Apple and virtually every other technology company will not just lose ground in this battle if the FBI succeeds — the ground will slide out from under them. Doing whatever is technically possible to stop terrorists will become compulsory. The idea of a technological backdoor will go from a controversial and almost universally despised idea to one that is a necessary evil — to combat true evil.”
See videos explaining the issue below: