The US alleges that Assange was part of a conspiracy to hack computers in the US, ie he was not just a passive receiver, he was involved in planning the hack.

The superseding indictment alleges that Assange was complicit with Chelsea Manning, a former intelligence analyst in the U.S. Army, in unlawfully obtaining and disclosing classified documents related to the national defense.  Specifically, the superseding indictment alleges that Assange  conspired with Manning;  obtained from Manning and aided and abetted her in obtaining classified information