FQ, There really is no legal or Constitutional prohibition against the FBI reading your mail.
The SCOTUS decisions about cell phone monitoring and tracking devices are based on "expectation of privacy".
The way I understand it, wire taps and hidden mics need a warrant, because inside your home or with a direct wire connection on a single party line you have a reasonable expectation of privacy, but when, as with Cell or radio communications, you are simply broadcasting on a common channel, that expectation goes away.
While the Constitution does guarantee your right to be "Secure in your person and papers" , that applies to the ones in your house, not so much to ones you drop in a box on some street corner, or leave in a box on the edge of the road.
Careful study would show that despite what people commonly think, while a "Democracy" may not have many options to protect itself from the type of subversion that has afflicted us, the Constitutional Republic We were intended to be does have the ability and authority to neutralize those who wish to destroy it.
The Constitution is not a suicide pact.