"If you copy something the original stays in place; it is not removed: no one is at a loss - how is copying theft?"
My answer to this was far too long to post on Twitter.
Before I am able to answer this question, I need to know: is the copied thing clearly marked as such, or is it treated in the same manner as the original? If the latter is the case, then I would argue that there is a loss: that of the benefits a creator is entitled to. To copy what someone else has made and claim it as your own is clearly theft; particularly if you use said copy as a means by which to benefit yourself.
At some point, this becomes a question of integrity more than anything else… often enough it seems the voices who most vehemently protest the right of anyone to own what they have created come from those who have never created anything of their own.