I appreciate what you are saying, but it is actually much more complicated than that, and it depends on what source you look to.
I am neither defending nor condemning piracy, but the semantics of whether or not it is "stealing" is not easily reducible to simple analogies about bookstores or cheeseburgers.
"Stealing" as a legal concept refers to larceny, conversion, or other forms of unlawfully obtaining objects or money. What we are calling "piracy" here (presumably not maritime piracy) is properly called "copyright infringement" and it may or may not be proper to refer to it as "theft".
The Supreme Court of the United States seems to have ruled that copyright infringement should not be called theft (see Dowling v. US), and this has been upheld in subsequent cases in which copyright infringement prosecutors have been prohibited from using the words "theft" or "stealing" in court. However, these semantic distinctions do not make copyright infringement any less illegal.
Meanwhile, the FBI has an informational page at
http://www.fbi.gov/about-us/investigate/white_collar/ipr/ipr that they call "Intellectual Property Theft" and indicates that copyright infringement is "robbing people of their ideas, inventions, and creative expressions".
So, long story short, it depends on who you ask, and there is no cut and dry answer as to whether "piracy" (copyright infringement) is or isn't "theft"