Specifying your license in package.json makes it a lot easier for your users to find and comply with GNU Affero licenses.
Also giving licenses (an array of objects with type and url keys) helps users whose legal departments request both the license name and the text. See this example from async:
Though licenses isn't specified in the npm package.json spec, it is supported by the NPM site, as you'll see if you check async's NPM page.
Sure, happy to merge a patch.