It seems like it isn't necessarily a good idea to let companies decide whether an individual request is justified. Suspects are innocent until proven guilty in a court of law. It's up to our society to remember that they are indeed innocent unless proven otherwise, and there's no way at that point for the investigator to prove anything.
Imagine that an investigator comes to Facebook and asks them for information regarding one of Facebook's employees. Facebook asks why, and the investigator responds that they suspect they're involved in something like what you've mentioned. At that point there's a chance FB might become extremely uncomfortable retaining the services of that employee, even though nothing has actually been proven yet. Accusations like that can ruin lives.
You make some good points, and it might be good to have more open communication between law enforcement and companies. It just seems a little dangerous. There are some unexpected ways that it could turn out to be a bad thing.