It's a slow system with a lot of flaws, but ultimately if Congress were to pass a law saying who the FBI could or could not use a 0-day against, they'd be legal

It's a slow system with a lot of flaws, but ultimately if Congress were to pass a law saying who the FBI could or could not use a 0-day against, they'd be legally bound to comply. Ditto for presidential directives. If you're arrested as a result evidence acquired through use of a 0-day, you can make an argument in court if you really think you have a compelling case. There's a whole lot of little details that make these easier or harder under certain circumstances, but none of these are impossible, especially if you have popular support for your cause. If you don't like the way something works, you can talk to your congressmen, vote, support organizations that promote your cause, etc. Just complaining on the internet won't change much.