Intellectual monopolies are not property
The rules that define the concept of property exist for a reason -- arbitration of rivalrous scarce resources. If you can imagine a magical world where cars or lawnmowers could be copied with the mere act of sight, you could understand how such a world would have no need for the concept of "property". But we do not live in such a world, so we must make do with rules that allow us to know who gets to control what at which time. So, whatever you obtained in compliance to these rules, you call "your property".
The reason why we have property rules obviously does not apply to intangibles. Intangibles are in principle non-rivalrous and non-scarce, just like the lawnmowers and cars from our magical world. Thus, there is no need for ethical rules to arbitrate the use of intangibles.
So, we say, laws notwithstanding, intellectual monopolies are not property, from any valid ethical standpoint.
What is theft and why is it wrong?
Moving on. Theft refers to the act of taking (via force or subterfuge) someone else's property against his wishes. Theft is a clear violation of the property rules we discussed in the last section.
The word "Taking" carries the implications that the property the thief takes is gone, thus the owner cannot use it anymore. Which is the whole reason theft is wrong -- after being robbed, the owner cannot use his property because it's gone -- exactly the situation that the rules of property are intended to prevent.
"Theft" does not apply to intangibles
However, in contrast to taking a man's property, copying an intangible always leaves the original. So, the premise that normally applies to taking someone's property simply does not hold for intangibles.
So the reason why theft is wrong and why we prohibit it, cannot rationally be used to prove duplication of intangibles wrong or to prohibit that act.
Thus, duplicating intangibles cannot possibly be theft in any ethical sense of the word.
How the idea that copying is theft came about
What the pro-intellectual-monopoly propagandists have done (spending tens of millions of dollars in the effort) is *pervert that implication*, by very carefully but falsely equating the act of copying with the act of taking, so that people build this false mental association between robbing (which everybody recognizes as an evil) and duplicating intangibles (an act whose prohibition is profitable to them).