An extradition is not a legal proceeding, it's a diplomatic one and being a democracy they can very well agree not to engage in a diplomatic exchange beforehand.
Well, I don't know what treaties are to the United Kingdom and Sweden, so I can't speak to the relevant matter, but there is a general sense (insofar as "international law" can be thought of as an actual concrete thing) that treaties are laws. To the United States, for example, treaties are explicitly treated as laws (see http://law.justia.com/constitution/us/article-2/18-treaties-...). Most other countries take their treaty obligations very seriously, although the actual mechanics depend on each country's legal/political system.
Source: http://sverigesradio.se/sida/artikel.aspx?programid=2054&art...
The allegations surfaced in August 2010, four months after Collateral Murder and one month after the Afghan War Diary was released. Also earlier that year the diplomatic cable releases began, and while most hadn't been released yet by August, some had, and they were a recently-known quantity by then.
The timing of the allegations was awfully convenient. Far more "convenient" than Assange's paranoia.
[1] http://www.bailii.org/ew/cases/EWHC/Admin/2011/2849.html
Weeeell, maybe because Sweden is a democracy and has no "authorities" that can guarantee the outcome of hypothetical future legal proceedings.
Also, note that Assange was obviously in Sweden when the rape allegedly happened, and only became conveniently afraid of extraditiom to US after those allegations surfaced.