I can't imagine that the public relations fallout from his arrest would be any worse than the public relations disaster they have with the release of these cables.
Are we still talking about the CIA? Has it become a law-enforcement agency and I didn't notice?
If your point is regarding the possibility of the US government going after Assange by legal means (not via some extralegal, shadowy CIA antics), I'm certain they would if they could.
The fact that they haven't leads me to believe their case against him isn't quite as airtight as you believe.
But regardless of what they do or try to do to Assange, it won't stop Wikileaks. The best that the US government could manage is to extract some sort of retribution or send out a warning to future leakers and whistleblowers.
I'm not sure whether he's broken any US laws or not. He did not, after all, leak the information, he merely publicised it after someone else (Bradley Manning, apparently) leaked it. Wikipedia mentions no warrants out for his arrest apart from the rape one, for which he's apparently right now "under arrest in absentia" in Sweden.
Bradley Manning, of course, remains in solitary at Quantico pending court martial -- he most certainly broke some laws.
If the US government (more specifically Obama's justice department) wanted to prosecute, surely they could, would, and would win.
There are political ramifications of that move. I would bet the negative (inflaming a lot of people against the Obama administration) outweigh the positive (not really sure what the positive is, the papers would still find their way out).
Thus, they are aiming their legal guns squarely at the guy who actually leaked this stuff in the first place.