Actually, he's not being charged with anything yet. The warrant is simply to require him to come to Sweden for questioning, after which time the prosecutor will decide whether to charge him with any crime. At the moment, he has only been accused of a crime.

Also, it is my understanding that the accusation was brought by the police officers after reading the statements given by the women in question. They had only come to ask if Assange could be required to take an HIV test.