Actually, if they claim to do it for God, but they really do it for other reasons, then it becomes a sin. However, if they are truthful of there full intent from the beginning, that's a little bit different. That's basically what Aninias and Saphira were about. They sold land, and gave money to the church, claiming it was the whole amount, when really they kept back a portion. Their goal was to achieve honor and glory by using God, and not giving glory and honor to God.
However, I'm not sure any artist who is considered a Christian artist wants there purpose to be considered anything but of God.