First hand I've seen crap thrown on the ice in "hockey markets" such as New Jersey, MSG, Montreal, Boston, Philadelphia, and Pittsburgh and in those cities all year hear is how "passionate" the fan base is. What happened was the DC fans simply had a temper-tantrum at what was idiotic one-sided officiating not only in Game 7 but for the whole series directly leading to the Capitals losing a Game 7 to a traditional rival in Philadelphia at home. I can guarantee you that the same thing would have happened in Newark if you had one goal coming if Marty had Oduya thrown into him and the GWG in OT scored after a mediocre tripping penalty after Devorski and Koharski swallowed their whistles turning the entire 3rd period into a hack-a-thon.
As to fan loyalties in DC they're no better/worse than anywhere else in the country. DC does have the built-in disadvantage of being perhaps the biggest transient city in the country but if the Caps are playing well they will draw. They just happen to have played like utter crap for a good 5 years and it takes time for people to become interested in the sport again. I remember when the Yankees were drawing flies winning 60 games a year back in the early 90's and George was screaming about moving to Secaucus.