There are two common comments against copyright: 1) Copying is not theft, 2) All art is derivative. The most common responses are: 1) Copying is not wrong on the same basis of theft, 2) This is why expressions but not ideas are copyright-able. I will focus on the first, here.
Different sections of intellectual property are founded on different ideas, but all have in common the goal of promoting useful arts and sciences (US Constitution Article I, Section 8). This “promotion” is primarily in terms of economic incentive. One argument in favor of copyright goes that by freely copying the works of an author or artist, we effectively rob the author of the money we would have paid for the work we got for free. [This raises the question: what if I was unwilling to pay for the art, but I am willing to accept it for free? This might describe my attitude towards a lot of vacuous, unimaginative, stupid, yet catchy pop tunes. If I was not going to pay in the first place, did I actually deny money to the author (and publisher, distributor, etc.) by getting her work for free?] Most theft does not seem to be a concern purely because a single object cannot stolen and remain in place at the same time. Suppose I could steal an object from a store, and yet the object remained there. The store still has it, yet I also have an identical copy of it. My ownership of the copy means I have no need to buy from the store, and so the store is economically harmed.
The argument that “copying is not theft” and is therefore permissible assumes the falsehood that theft is wrong because it deprives someone of the object being stolen. In fact, theft can be wrong because it deprives someone of what the object represents or leads to (such as economic benefit).