If you're trying to get hired there, it may be somewhat reasonable. In the interview process, you might see or hear some things that they don't want the world to know.
If you're visiting to do business with them... I like generalpass's answer. They can demand an NDA, as a condition of doing business, but that's not just a condition on you as a person - it's a condition on your whole business, and you need guidance from your business to know how to handle this. At that point, it's a negotiation between the two businesses, and not your problem.