It's not that complicated really. He went to renew his Kuwait visa and was detained and interrogated. Kuwait decided they want to deport him back to the US. As such he's going to be detained until either he's deported or they change their mind. The US claims that he cannot be deported because he's been added to a no-fly list (obviously, that has happened at some point recently).
He can't walk freely to a harbor and board a ship. He cannot be deported because the US is barring him entry. He's effectively imprisoned until the US removes him from the no-fly list or Kuwait decides to grant him a visa and release him.
Let's not forget for a moment here that Kuwait, as Greenwald points out, is basically a country that does whatever the US tells them to do.