By definition, any law passed by a country is legal.
That was the first charge brought against him and it alleges that he helped Chelsea Manning crack a password hash, to allow Manning to use a different user name to download the documents. Whether or not he did it, is something that the Courts would need to decide. As it only accounts for five years of the 170 he faces if convicted of every charge, I doubt it is anything to do with plea bargaining.
If that were true, the extradition proceedings would already have failed.