Sure, the US does want to question him. But they haven't issued an extradition request, and by the looks of things getting him out of Sweden would be just as difficult as getting him out of the UK, if not more so.
I highly suspect Assange is actually just trying to avoid jail time, if he's going to these lengths to avoid going to Sweden for questioning on unrelated accusations of sex crime.
Assange is very attention-seeking and seems to care a lot about his image, which I suspect he is going to great lengths to avoid damaging.
Edit: I hate to compare HN to reddit, but I worry that I'm being downvoted for having an unpopular opinion. (edit: this has changed.)
Edit 2: I'm reconsidering my stance on this, my father's (conservative) opinions have too much influence on me.