Well....we don't know the circumstances. 

If they were totally unaware about the need for a license....and the the officer did not simply inform them that they needed such, and told them to "cease and desist" until they had one  (i.e....cuffed them right off...with no discussion)....then sure....they were being unreasonable. 

HOWEVER....it's a real possibility that the officer had seen them there one or more times before.....given them such information and a warning (perhaps several)...and they kept sneaking back.  Or....may have initially informed them of the need for a license, etc.  and they either ignored the warning or became belligerent, causing the situation to inevitably escalate.

To be honest, I'd rather not have our National Mall being packed with shoddy vendors everywhere, like some Caribbean tourist trap. 

Without knowing the details, this criticism may be (in this case) unfair.

**And I say this as a bureaucracy-loathing staunch conservative.**