Well according to dictionary.com theft is "the act of stealing", and steal has a number of definitions including "to take (the property of another or others) without permission or right", "to appropriate (ideas, credit, words, etc.) without right or acknowledgment", and "to take, get, or win insidiously, surreptitiously, subtly, or by chance". I think the definition is appropriate for piracy: technically you are copying or using digital work without permission (more broadly speaking "appropriating ideas"), or "taking insidiously". The fact digital goods can be easily copied is simply a detail, and I think the image was prepared by trolls trying to defend their use of piracy by misleading people away from the dictionary definition.