First: D.C. is a special case when it comes to domestic use of the military. The elected official in D.C. with the proper authority to use the National Guard is the president, not the mayor of D.C. Everywhere else in the mainland US, the governor is so empowered. In D.C., it is the president who is so empowered. The President is basically the governor of D.C. I don't believe a request from the mayor has ever been required for use of the D.C. Guard. The D.C. Guard answers to the president, just like the state Guardsmen answer to the governors of their respective state (unless they are nationalized, in which case they report to the president).
There are other precedents. The Arkansas governor used the Arkansas Guard to block black students from schools, and Eisenhower used the Regular Army to reverse that. Kennedy nationalized the Alabama Guard to quell riots, in direct opposition to Gov. George Wallace.
Also, I think the Army has always stood ready to guard the White House.
So, I'm not sure this is a breach of U.S. norms.
Second: These rioters are advancing on the White House, when the President has nothing to do with what they are protesting. They weren't going to the Mayor of D.C.'s house. Or the Mayor of Minneapolis house. They were advancing on the White House, setting buildings on fire. Can we assume their intentions were noble? I don't think so. I can see how the military was ready to assist the Secret Service (many of whose officers were wounded).
You said "quell protests." But, the protests had stopped, and the riots had begun, had they not? I confess I didn't research the timing, but my impression was that the riots had begun.