He also promised to take a principled stand against torture and to close Guantanomo, and restore due process; before being elected.
Since being elected, none of those things have happened, and he has come out in favour of assassinating US Citizens living abroad without any judicial process whatsoever (The Al-Awliki case).
So the failure of Obama to publicly condemn Lieberman's petty campaign of censorship and crude threats against Amazon is not at all surprising.