The words have a specific technical meaning in this case, so it's important to use them correctly. A robbery and a simple theft are treated very differently under the law. Prescriptive grammar is the only way to go when you're dealing with a technical system built out of ordinary language, like law or programming.
I think that's fine in general. However, in this case, "robbery" is basically being used as a technical legal term describing a particular crime. It's a little bit like calling every laptop a macbook (except there's no trademarks involved).
