I am not assuming the FBI doesn't investigate random people. I am, however assuming that the FBI does not randomly seize computers and obtain court orders demanding encryption keys for them from Microsoft. Unless Microsoft is lying, that happens about 20 times a year.

One of the privacy protections is simply that it's a lot of work to go through that process. The FBI wouldn't have the resources to do it to everyone it's merely curious about even if it had the authority, which it doesn't because warrants require probable cause.

I believe that it's generally acceptable that when law enforcement has probable cause for a search warrant, third parties grant them what access they reasonably can. I also believe people who actually want to protect their privacy and security should learn fundamentals like whoever has the key can unlock it and if nobody has the key, it's gone forever. If I was building a consumer product, I'd have to care quite a bit about the fact that many people won't do that, but I'm not so I don't.