There's two big points to using a VW/Audi (or Honda, Toyota, Ford, what-have-you) motor in something like this: emissions compliance and availability. The major maufacturers would have already done all the heavy lifting in terms of development and government certification, and it's far more practical to buy a motor off the shelf than to spend what would be needed for an in-house creation, especially for such a limited production run (too limited, IMHO).
Best example: the Toyota motor in the Lotus Elise. It wouldn't be over here otherwise.