Thanks to this article, I can finally put into words what bothers me about "Copyright infringement is theft"!
Yes copyright infringement share some of the properties of theft, but they are sufficiently different that we shouldn't apply our intuitions about theft to copyright infringement, just like we shouldn't apply our intuitions about theft to taxation.
When someone says that piracy or taxation are theft, they are not necessarily appealing to intuitions about theft. They may literally hold and mean to state that they share with carjacking whatever makes carjacking theft, WITHOUT sharing other properties of carjacking... in effect, this is a thesis statement rather than a knock-down argument.
Of course, it is also possible that it is not even an honest appeal to intuition, rather just an emotional tarring which will never be substantiated. This is usually what is irritating about hearing those arguments.
Yes copyright infringement share some of the properties of theft, but they are sufficiently different that we shouldn't apply our intuitions about theft to copyright infringement, just like we shouldn't apply our intuitions about theft to taxation.