Given the way the US government has behaved over the past ten years, the fear is not irrational. Also, we're talking about the possibility of extradition to the US from Sweden, not from the UK.
Yes but if the US want him they can get a US judge to decide he has a case to answer and then seek his extradition from the UK, they don't need to go through Sweden.
"Throughout this process have we have drawn the Ecuadorians' attention to relevant provisions of our law, whether, for example, the extensive human rights safeguards in our extradition procedures, or to the legal status of diplomatic premises in the UK," the spokesman said."
Perhaps Sweden has fewer safeguards built into its extradition laws?
What happens if the US has already secretly convicted and sentenced Assange in his absence? Maybe even the death sentence? It might be the case that the UK's extradition laws with "extensive human rights safeguards" would prevent extradition to the US, but Sweden's laws wouldn't. Extradition to Sweden is a way for the UK to step around their own human rights laws.
http://www.guardian.co.uk/commentisfree/2012/jun/20/julian-a...