DC market is high pressure and very competitive. Also, it seems to be more towards the commercial sector and DOD. Of course the government stuff is off limits for showing around and in my field, cases have to be settle first or have gone to trial. Even then, our clients have strict rules that have to do less with restricting an artist, and more with preventing future issues or even a public relations mess. When a company spends large sums of money in branding and corporate id, they tend to want to control anything that is in the public eye.
The only other thing I think that companies want to avoid is an artist showing a clip of work and getting a large account versus just getting a job. Many employees have been known to quit and "open up shop across the street" as they say. Sometimes they are concerned about you walking to a competitor with the company's proprietary information and any competitively advanced techniques. Of course the paranoia is only there because the best talent to recruit is the one someone else already spent the money to train
and on top of that, when you go to a new company, many take clients with them or the new employer will ask if you have any leads.