Paul, you're wrong.
Calling...
http://developer.echonest.com/api/v4/artist/genres?api_key=API_KEY&name=weezer
Returns...
{"response": {"status": {"version": "4.2", "code": 0, "message": "Success"}, "terms": {"genre": "rock", "sub_genre": "alternative rock"}, "id": "AR633SY1187B9AC3B9", "name": "Weezer"}}