Now now, the NSA are not reading the letter.
They're intercepting the letter, opening it, photographing the contents, scanning it for OCR for text-based indexing and saving metadata about who the envelope says sent and received it, how many words there are, topic domains and so on for searching, then carefully sealing the photographs away, unread by humans.
If a later search across the metadata suggests there might be something, or if a full-text indexed search suggests there might be something, then they have evidence with which to establish probable cause to get a warrant to unseal the photographs of the letter you sent. Because they've successfully argued to the Star Chamber, er I mean FISA, that the interception doesn't _really_ take place until a human looks at the photos, they argue that this is all legal and compliant with the US Constitution.
And folks? Echelon has for decades operated on the principle of "we spy on your folks, you spy on our folks, neither breaks the letter of the law and then we just hand over the data to each other", in the USA treating the US Fourth Amendment as an inconvenient problem to be worked around, and the Bill of Rights as a subject of contempt, instead of something to be safeguarded.
The difference is that with PATRIOT the NSA was able to start cutting out the middle-man. If GCHQ are still involved at all then it's just because the NSA don't want to burn any bridges and want an ally to fall back on if PATRIOT ever gets repealed and they have to go back to the Old Ways.
Good to see The Guardian continuing this battle. It was 20 years ago that I first read (in The Observer?) about the Echelon stuff, and the British "Class Warrants".
Seriously, if folks in the UK are going to get upset, you should be upset about the continued existence and use (and lack of repeal) of Class Warrants. What is it, 98+% of the UK population covered under Class Warrants that have never been repealed? So if you own or have ever owned a motorcycle, then as a Dangerous Person the UK security services don't _need_ an individual warrant for you. You're already covered under a Class Warrant.