NPM included their terms of service into the open source Artistic licence without letting anyone know.
Every download of node 4, the 4 LTS and node 5 is affected and has this version of the license. So it seems you may have accepted the terms even if you only use the client. NPM wrote into terms you accept changes without notice so you don't really know what you have agreed to.
No one knows what this means legally yet. NPM's lawyer made a comment on this but it needs a lawyer that is not NPMs lawyer to provide an opinion.
NPM is also changing its terms and adding more restrictions on users. It also wants a free license to do whatever it needs to with your software. There are concerns from commercial developers including StrongLoop.