People may find arguments about semantics silly, but they are important. Just take a look at the whole brouhaha about evolution just being a theory: a large part of that whole disconnect stems from two meanings of the word "theory" that are not synonyms but are still very close. This is exactly the problem we want to avoid with "theft"--calling copying theft says as little about it as calling evolution a theory.
That is, there is either the proper meaning in context (e.g. infringing copyright) which does not mesh with the common definition or there is the common definition (depriving somebody of property) which does not mesh with the context. People framing copyright infringement as "theft" want to take advantage of emotional connotations of the word in much the same way as people capitalizing on the fact that evolution is "just a theory".
That is, there is either the proper meaning in context (e.g. infringing copyright) which does not mesh with the common definition or there is the common definition (depriving somebody of property) which does not mesh with the context. People framing copyright infringement as "theft" want to take advantage of emotional connotations of the word in much the same way as people capitalizing on the fact that evolution is "just a theory".