There is a BIG difference!! Ferguson,Baltimore were NOT Federal offenses. Washington D.C. IS as they were an attempt to disrupt a Federal function i.e. Presidential inaugeration.The others were state offenses.
So rioting and property destruction is legal in the states? Are there no felony rioting laws in the states? They can be enforced just as well at a lower level.
Given the extreme left city governments in Ferguson and Baltimore,it doesn't much matter to them WHAT the law is unless it fits their agenda. I doubt the D.C. CITY government would be likely to be so hard on rioters, but this is out of their hands due to the event.