Unfortunately for Julian he is not a Swedish citizen.
Since Julian has yet to be charged with anything and the Swedish authorities apparently just want to talk to him why can't they get on a Ryan Air flight and come and talk to him in London? Provide a rational answer for this question and I will believe that Julian is not looking a one way trip to the USA.
Purely political. To make a statement.
I can of course not prove this; nor have anyone else been able to as far as I know.
But, that said, I think it's a non sequitur that the current situation, which at least in my mind can only be understood by either absolute incompetence from the prosecution or being under political pressure, would also lead to extradition.
I would absolutely go so far as to say that extraditing him from Sweden would severely impact the relationship between the US and Sweden.
If police ever indicate that you're wanted on suspicion of a crime, try telling them you'll agree to help them with their enquiries so long as they agree that you set up the interview environment and they can't take you into custody afterwards. I guarantee they won't accede to your request, even if you've never leaked a diplomatic cable in your life.
Now provide us with a rational answer as to why those same Swedish authorities let him walk around freely and even leave the country after tipping him off about the allegations if their primary motivation was a one-way ticket to the USA?
Regardless whether or not the motivation is a one-way ticket to the USA, the answer to why they let him walk around freely and leave the country is simple:
It was not the same people.
The current prosecutor intervened in highly unusual manner after the previous prosecutor had indicated there clearly was no crime.
Whether or not that is because of US involvement or, say, her own personal political point-scoring is another matter, but to present it as if it was the "same people" driving the decision to free him initially and the decision to pursue him afterwards is misleading at best.
Even if she couldn't have intervened in earlier decisions that went the other way, this doesn't seem like the course of action that a prosecutor motivated by the desire to restrict Assange's freedom of movement would take...
If she was merely told it would be beneficial to her career if she helpfully found a way to get him locked up, and others behind the scenes prepared to smooth the road towards getting him to the US, then it's not so strange.
She could have rattled a lot of cages and brought him in right away, but then they'd also need to be prepared to handle much more fallout.
Note that I'm not saying the US is behind this. I don't think it's the most likely motivation between Ny's strange behaviour in this case. But I also think that the two prosecutors different opinions about the case is the easy explanation for why he was allowed to walk free to the extent that he got the opportunity to leave before they were ready to arrest him.
So in order to dictate "how and when" one can be questioned by the police, as a fugitive you first have to get political asylum in a other nations embassy, sit there for 3 years, and then have the statue of limitation reaching into the last 3 months, and the total cost of the police operation should be around $10 millions.
The risk that this "precedent" should impact even a single fugitive interview in the next 100 years is likely to be small, or actually very small.