I'm not sure that's true.
JUNE 7--A high-tech computer system that captures images of “every mail piece that is processed” by the United State Postal Service was critical in helping federal agents track the Texas woman arrested today for allegedly sending ricin-tainted letters to President Barack Obama and New York City Mayor Michael Bloomberg.
http://www.thesmokinggun.com/documents/woman-arrested-for-ob...
The technology for doing this in a virtually undetectable fashion has been around for a long time, and I'm sure it's far more advanced now. I very much doubt that ordinary letters have to be opened to see their contents anymore. It can probably be easily done using any number of scanning technologies.
The main problem for them is that it takes more time, effort, and money to do this for snail mail than to just ask a company for a hard drive containing all the emails and tweets of a few hundred million people.
On the other hand, a lot fewer people are using snail mail to communicate these days. So total surveillance of the relatively small amount of snail mail might just be done as a matter of course anyway.
Of course, there are some pesky laws presumably protecting the sanctity of physical mail. But if you think spies have ever let the law stand in their way, you live in a much rosier and prettier world than I do.