Self-preservation alone would be enough I'd argue.

It's a hideous and dangerous precedent even in a long and terrible history of dangerous precedents. And Assange was a relatively well connected, broadly (at the time, as far as most were concerned) apolitical figure. If the US had been able to walk through what they wanted, with little to no concern or push back from other states and legal groups, I'm certain they'd already be black bagging and trying left wing journalists and whistleblowers of much lower profiles in secret military courts.

I don't think this plea deal or any amount of public outcry is going to stop the likes of the CIA pulling that kind of thing if they really want or need to enough. But it does make it harder. It does indicate there's stakes for governments who go along with it. It does add a cost to that kind of activity. And it may have helped stop it becoming policy more broadly used.