Was flying a helicopter below building height against FAA regulations?
I don’t know exactly where in DC that happened, though I would say it’s highly probable that it was a public or law enforcement helicopter, or a lifeflite helo as virtually nothing else is allowed to fly VFR in the FRZ over DC without a lot of planning and waivers.

Helicopters, under general operation, are going to have to conform with §91.119(a), (d)(1), which state you must operate the helicopter at an altitude that, if a power unit were to fail, a landing can be made without creating an undue hazard to persons or property on the surface.  In the case of the helo in question, it’s a Eurocopter EC 135, which is a multi engine aircraft.  I don’t have the performance charts for that helo handy, but I believe it either can hover OGE or perform and aurtorotation landing at near standstill should an engine fail.  

There’s also the fact that, as many cities right now are under a state of emergency, public vehicles may be forgoing a lot of regulations on flight ops for that reason.  The helo may also be looking for a place to land and hoping the people below can make a space for it.  I’m not sure here.