From a technical standpoint and a legal one, piracy is copyright infringement, not theft. It's a similar behavior but fundamentally different altogether at its core, largely because the victim of piracy does not lose anything tangible, only a perceived opportunity of a sale (which can be argued as not having been an opportunity in the first place depending on who is performing the piracy).

Both are bad, but people calling piracy theft have a terrible misunderstanding about it.