so might you or any other passenger
yeah but the authorities need some kind of "probable cause" (that's not the right phrase I'm sure). They can't just stop people at random, they need some kind of plausible reason to believe the individual should be stopped.
Miranda assists Greenwald, it's a reasonable position that he may be carrying these things, he has a connection to them.
I'm just a random bod Joe Schmoe.
If he was a spy or a journo with these documents would he really carry them through customs when he could have stored them electronically?
well ... can't email them because emails will be intercepted. Can't carry hard copy presumably ... very bulky and can't be encrypted. Can't whack them up on Google Docs or Dropbox, likely the NSA has copies of all of those. What's left ... carry an encrypted copy digitally on a hard drive or URB drive.
As for customs, if he was just changing planes he wouldn't go through customs would he? (Genuine question, it's been years since I've done it).
ps personally I'm neutral on whether this was a valid thing to do or not, I don't know. Time will tell, and us bods don't have the full facts. I'm just making the case it might be, especially to somebody whose job was working for UK government security. So far the "definitely out of order" camp haven't convinced me.