Because terrorists use the US Post Office to communicate, of course. From the New York Times:
WASHINGTON — In a rare public accounting of its mass surveillance program, the United States Postal Service reported that it approved nearly 50,000 requests last year from law enforcement agencies and its own internal inspection unit to secretly monitor the mail of Americans for use in criminal and national security investigations.
The audit found that in many cases the Postal Service approved requests to monitor an individual’s mail without adequately describing the reason or having proper written authorization.
In a show of bipartisanship both Republicans and Democrats support trampling on our Fourth Amendment rights.