For copyright, the act of me creating something doesn't deprive you of anything, except the ability to consume or use the thing I created. If I were influenced by something, you can still be influenced by that same thing - I do not exhaust any resources I used.
This is wholely different from physical objects. If I create a knife, I deprive you of the ability to make something else from those natural resources. Natural resources that I didn't create - I merely exploited them.
Because of this, I'm fine with copyright (patents are another story). But I have some issues with physical property.