When he leaves he will be transferred to the US and tortured, just like Manning was. His trial will be a farce, just like Manning's (did you know POTUS—Obama at the time—declared Manning guilty prior to trial?). You, or I, or anyone would skip out on bail if appearing means you get given to torturers who will torture you without a trial. That's not his choice, it's the choice of the people out to physically harm him.

If he stays he will never receive a trial or be allowed to face his accusers.

Either way his human rights are violated. That's why it's arbitrary detention.

It's why his indictment has been kept secret; they want to physically grab him before they have to justify, legally, the charges against a journalist who published things in the public interest. If they weren't being shady about the whole thing, he could have been charged by now - but doing so would require them to be subject to legal scrutiny. They don't want that, because they don't have a leg to stand on. They won't tip their hand officially until he's already physically in their (or their allies') custody.