In the former case, in the absence of the law, the act is still wrong. In the latter, the act is not wrong except that it is thought to be detrimental to the balance required to maintain a productive society. However, that opinion may be wrong, since there are many ways a balance can be struck, so bible-thumping about ownership rights to works (which is a convenient fabrication anyway - nothing is owned in the case of copyright) doesn't really serve your argument, since you are using the definition of copyright to justify itself.
If you really want to be convincing, why don't you argue for what you feel that copyright should be? Infinite rights including limiting ownership rights? Infinite duration? Private policing powers? Or limited exclusive rights for a limited duration?