In his lifetime, Assange has hacked the Pentagon, US Navy telcom systems, other DoD infrastructure, Lockheed Martin, Panasonic, and others through a hacking group he founded[0]. That's not even including Wikileaks and his suspected ties to the Russian government.
Combine that with the fact that most countries carry extradition treaties, it's not all that surprising that the US wants him. He's committed a litany of crimes in his lifetime against foreign countries, and often bragged about it. Why is it crazy to you that a foreign country wants to detain someone that has hacked said country's critical military infrastructure and participated in information laundering that potentially impacted a US presidential election?