You can assume no such thing. In general, legal systems don't do negotiations with people wanted for questioning. Assange has come up with a continuously changing list of excuses why he doesn't want to go to Sweden to answer questions about rape charges, and the excuses evolve to fit whatever he seems to think will best please the audience. Since he could end up facing rape charges, one can see why he might want to not visit the police in Sweden. Possibly he should go to Switzerland, where he could join Roman Polanski, also fugitive from rape charges.
I can assume anything I choose to: It sure seems awfully convenient that just a few weeks after Wikileaks exposes war crimes by the United States that two women affiliated with the CIA file accusations of rape against the leader of the group.
Bottom line: The prosecutor closed the case in Sweden prior to his release and being allowed to leave the country. He turned himself in and was already questioned and released. What probative value does the prosecutor believe will emerge here from this re-opened investigation? Zero new details have emerged: What purpose (other than setting up the accused for extraordinary rendition to the United States) would it serve? They were offered a number of options to interview him in England and they declined. He declined to voluntarily travel to Sweden after they refused to guarantee he would not be "extraordinarily rendered" by the United States.
So I guess I'd say: If these people were really interested in "justice" and "Getting the facts" they'd have gotten on a plane and interviewed him in London.
And I'd say you're naive if you think prosecutors don't negotiate terms for voluntary interviews--they do it all the time. It is standard procedure for getting people to answer questions. Don't like it? Too bad: That's life.