I've been contemplating on this for several years already. No one could give a clear answer. What I have concluded is that this question only shows the overall sillyness of digital copyrighting, because essentially digital information does not belong and cannot be forbidden to reproduce. The instinct of ownership is not really applicable in the digital world, and the desire to grab and claim for yourself as much as possible does not really work here. Really, you can have authorship, like in science where theoremes and laws are named after people who first presented them to the public, that's all.
In fact, any lawyer can accuse you as well as justify basing on his personal mood and opinion. After all, lawyers are never scientists, so how can they know that they misunderstand something fundamental like math or physics.. but it is funny that these people will be the ones to effectively judge you. To be blamed for having a sequence of bytes is like being crucified and burned for saying that the Earth is a sphere that rotates around the Sun in the past times. At the same time anyone who understands the science enough can never blame you in the name of digital copyright. But that does not save some people from the craving that someone else is having good times with their sequence of bytes, and that's where lawyers will come to his aid, take his money and just do what they do, like mercinaries.
The notion of prohibiting copying comes from the instinct of ownership when there was only stealing and no copying. Like, you can steal someones plant, that's bad, but what about growing a similar plant? That was never in question.
Prohibiting this is like prohibiting a sequence of actions because this particular sequence is reserved for someone else, not because it brings in some real problems. Imagine that you have found out that drinking coffee with a sandwitch is tasty and joyful. Imagine you will be arrested for doing this because some big restaurant patented this as their sequence of receiving a particular result.
So the ultimate answer to this question is that our society and economy are not yet ready to realize the truth of digital information and there is no point at which such a file becomes illegal. And there cannot ever be any social law that will adequately define such a point - you might just as well define it as arbitrary as you want. The only factor that matters is the personal opinions and attitudes of the involved lawyers and their ability to tie it with the existing state laws.
A nice quote by E. O. Wilson fits well: "The essence of humanity's ... dilemma is that we evolved genetically to accept one truth and discovered another."