When I worked for a very large networking company, we wanted to bring in a small company's only product which would have been central to the day-to-day running of engineering. Because they were so small with so little track record, we wanted to make sure that we could get their source code should they ever go out of business.

I can't think of a good reason to require code inspection prior to purchase. Testing, measuring, and generally trying to break the code during evaluation seems fair game though.