They did something similar with Chelsea Manning, announced they were "thinking about a pardon" to see if there would be pushback.
It's proving to be rather embarrassing for the UK, it's clear to many here how he's been treated and how much of a stitch up the trial has been. I imagine the US could do without the international attention on this - court cases, more Russia speculation in an election year.
And the "authorities" have made their point: don't try to take the piss out of us, we will ruin your life even if we don't get you to trial. Julian Assange's case will certainly be putting off quite a few people from becoming radical journalists.