Copying is not theft because there are no such things as property rights in the value of things. Value is the valuation of others, it exists in the minds of other people. You do not have a property right to the value of your house, for instance, in the sense that you can't have a right to how other people think (or how other people evaluate).

Creating copyable things does take work and it is the right of a creator to be able to sell it. It would also be his right to sign a contract with his customer not to copy the work. DRM, watermarks, SaaS are all good tools to reduce unauthorized copying.

With that said, even if you disagree about copyrights, can we not call it a property right?