The destruction of the Guardian's laptops was about ensuring they didn't get stolen by someone else - if you read the story at the time, the spooks actually wanted the Guardian to hand the laptops over but Guardian refused and destruction was a mutually agreed way out.

Miranda's detainment, confiscation of the memory sticks etc was to be expected - as far as the UK Government is concerned he's carry stolen state secrets.

What I don't really understand is why he flew through London carrying them, I believe Madrid has more routes to South American - I wonder if he was routed so he would be picked up for massive publicity.