So there's two possible reasons for them stopping him and both are kind of pathetic. One, that stopping someone for 9 hours in Heathrow really amounts to much intimidation of anything. I mean it's wrong and wrong headed but I struggle to buy that someone with half a brain thought this would be at all intimidating. I mean this is England for crying out loud. They probably spoke loudly at him and refused him tea.
So I lean toward they really wanted to see if they could find anything on him directly or indirectly that would help them get to all those documents Greenwald is now saying he'll release. But I understand that most people will chose between these two reasons and it's hard to prove one or the other.
If nothing else, it's intimidating because if they can detain you under a terrorism law for doing journalism, it stands to reason that they can bend or abuse some other over-broad law to actually charge/imprison you depending on the interrogation. Also, you get to hand over every piece of electronics you're carrying for an indefinite period of time. If and when you get any of them back, you might as well assume they're bugged or otherwise compromised, so they're essentially useless unless you want to try your hand at finding any backdoors that might be there. Fun.
I would caution against presuming that nine hours of detention in an interrogation room with a team of very well trained, highly motivated interrogators couldn't be extremely intimidating - regardless of location.
So I lean toward they really wanted to see if they could find anything on him directly or indirectly that would help them get to all those documents Greenwald is now saying he'll release. But I understand that most people will chose between these two reasons and it's hard to prove one or the other.