https://www.schneier.com/cryptography/paperfiles/paper-keys-...
Note that all the techniques they have allow stealthy, persistent access with ability to forge evidence with write access. I don't have to be libertarian to be concerned about that given all the corruption cases of local and federal law enforcement. Just know human nature & scope of technical problem.
In the case of a cloud service provider, putting the company (and a judge) in the loop is a procedural speedbump. The government doesn't have write access unless the company allows it. Of course if they collaborate all bets are off, but that's the best that separation of powers can give you.
Far as write or collaborate, you should look up the Lavabit case records or summaries. The FBI and court demanded that he hand over the master key compromising all users then lie to them about it. FBI also wanted to put a black box in a privileged position that could probably be used to compromise the service. They've used 0-days before in ops.
So, a backdoor with enough privilege to read everything in the system, install updates with their spyware, and be unnoticeable to OS must be both (a) enormous technical risk and (b) have write access that could enable corrupt officials to frame dissidents. I've worked on potential counters to A but B is unacceptable given fed's and spook's track records.
In the happier case (with better lawyers), a legal procedure gets worked out where the company checks that the search warrants are valid, law enforcement gets the info they want, the company keeps their private key, the government doesn't get to install any hardware, and companies publish "transparency reports" showing that mass surveillance isn't happening.
So, you are told that it happens as you describe. We cant be assured of tgat given leaks.
If you're not a libertarian, it matters that law enforcement has to get a search warrant, rather than breaking the encryption and snooping on whatever they want.