You could argue the opposite side: how many embassies offer protection to famous rape suspects?
This is not an ordinary occurrence.
The only reasons you would want a suspect extradited to your country are: to press charges and make an arrest, to torture, to make a media scene, or to 'punish' someone by detaining them even though you can't make the case stick.
The only acceptable reason is the first one, in which case you should press charges before extradition.
At this point in the US he'd probably be charged with obstruction of justice, at which point venue is again no longer up to him.
In the US there's the 5th Amendment to the Constitution.
Even if you know nothing of the law, surely you've at least seen a television show or movie where someone says "Am I being charged with a crime? ... Then am I free to go?"
That's all you need to say.
Assange has a perfectly justified fear that the request for interview functions as a strategy for entrapping & extraditing him. The notion that this is nothing more than a disinterested preference in procedure is implausible.
This is true in the US. It's not quite true here in Australia - a refusal to talk to police can be used against you in a court of law. It's not going to make or break a case, but you can't just 'plead the 5th' here and have no consequences. I imagine it's similar in the UK as our law is quite similar to theirs.
"You do not have to say anything, but it may harm your defence if you do not mention, when questioned, something which you later rely on in court. Anything you do say may be given in evidence."
http://en.wikipedia.org/wiki/Roman_Polanski#Sexual_abuse_cas...
Which is why I wrote: This is not an ordinary occurrence meaning that it happens rarely, not that such things never happen.