It really comes down to a question of if there are sufficient copyright related questions to justify the tag's existence. (I have a feeling there may not be.) While I can think of a lot of law related questions, the number of questions about copyright is far more limited. If we get rid of it, we will certainly need to keep law. There is no way the two are synonyms and law is the most inclusive.
Copyright also is interesting in that it could have some ambiguity. It could deal with both the legal side of legal obligations towards removing copyright content, but it could also deal with the licenses and copyright of content that is posted in a community as well. This later category might not fit quite so well in to the law tag.
I guess my initial impression would be to keep both for now unless copyright ends up being underutilized.