What pressure does a statement from the man himself apply to the US? I fail to see how this improves his situation one whit.
As someone else said, remaining silent lets his opposition control the discourse. You can see this clearly in the frenzy of stories in the media about Snowden's stripper girlfriend and the "IS HE A COMMIE SPY?" headlines. Providing a counterbalance makes it harder for the government propaganda machine to sway public opinion against him and turn him into Just Another Terrorist.
A 'counterbalance' can often instead be a distraction which is what I fear happened here.