As a company you'd be asking for an internal implosion of your company if everything had an additional backdoor in it.
Any backdoor added is a backdoor the thousands to tens of thousands of advanced hackers are always actively trying to breach. So if they do it they'd be doing it very selectively via special served code.
It's also the only way they'd stop whistle blowers.
Now that I say it. That's 100% what they would do. But again it's a crazy high risk almost zero reward action for them. Is the CIA paying apple a bajillion dollars for phones? No so why unless their arm is twisted would they risk billions of dollars for basically no gain?
This is not bulletproof, because they can potentially change the code to do this - this is what the FBI vs Apple thing was a few years ago was all about.