I think that you've taken this too far. We can't make blanket statements about peoples rights and expect them to be applicable in every situation.
In this case, a legal authority made the decision to interrupt a service for a small period of time in order to investigate a terrorist act that was disrupting the lives of thousands of people daily. If this was, say, a service that was providing medical assistance to elderly people, I don't think the same decision would have been made.
I'm a student at the University of Pittsburgh. From that point of view, this seems like a perfectly fine action to take. The FBI agents may have disrupted service to a few hundred users, but these threats were affecting thousands of people.
I don't know if these anonymous email threats were unprecedented or not, but they certainly present a huge challenge to law enforcement. What would have happened if the people responsible didn't just stop?
Of course, you could subscribe to the view of the conspiracy theorists among us who feel that the FBI did actually catch the people responsible and are now employing them. :P
In this case, a legal authority made the decision to interrupt a service for a small period of time in order to investigate a terrorist act that was disrupting the lives of thousands of people daily. If this was, say, a service that was providing medical assistance to elderly people, I don't think the same decision would have been made.