Out-of-state National Guard units were used in Washington, DC during protests. This law professor wants to know why

During recent George Floyd protests in Washington, D.C., members of the National Guard included not only the D.C. National Guard, but also, National Guard units sent in from at least 11 different states. Legal expert Steve Vladeck, who teaches at the University of Texas Law School, wants to know why and examines the use of non-D.C. National Guard units during those protests in an article published by Lawfare on June 9.
“Over the previous week,” Vladeck explains, “thousands of National Guard troops from states across the country arrived in Washington, D.C. as part of the Trump Administration’s response to the largely peaceful protests taking place across the city. After a great deal of controversy — including an argument over whether troops were allegedly kicked out of their hotels by D.C. Mayor Muriel Bowser — they have now departed back to their home states. But under what legal authority were they deployed to D.C. in the first place? The answer was not obvious, and the (Trump) Administration initially remained silent as to its reasoning. Now, in a letter to Mayor Bowser, Attorney General William Barr has cleared up that mystery, explaining that the out-of-state National Guard troops were there under the authority of 32 U.S.C. § 502(f).”
Vladeck points out that in the United States, National Guard units can “wear three different hats.” Hat #1 is “state active duty,” which Vladeck describes as “exercising state functions at the request of the state government.” And Hat #2, according to Vladeck, is “Title 32 status” in which “state National Guard troops remain subject to state command and control but are used for federal missions authorized by Congress — and, perhaps most importantly, are usually paid for by the federal government.”
Hat #3, Vladeck writes, is “Title 10 status” in which “state National Guard units are ‘federalized’ by the president of the United States pursuant to one of the specific statutory authorities for doing so. Once federalized, National Guard troops come under the full command and control of the Pentagon —specifically the secretary of defense. In essence, National Guard troops become part of the federal military until and unless they are returned to state status.”
Vladeck notes, “It’s long been understood that the Posse Comitatus Act — which prohibits ‘(using) any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws’ — does not apply to National Guard units in either SAD or Title 32 status, because they are not, at that point, part of the Army or the Air Force. By contrast, when National Guard troops are federalized, the Posse Comitatus Act does apply — and requires ‘express’ statutory authorization before those troops can be used to “execute the laws.’”
Vladeck concludes his article by weighing two possible scenarios.
“Ultimately, one of two things is true: either § 502(f) does allow the federal government to use out-of-state National Guard troops as it did last week in Washington — for any purpose and under federal control — which is deeply concerning and crying out for some kind of legislative reform,” Vladeck asserts. “Or it doesn’t, and upward of 5000 out-of-state National Guard troops were unlawfully deployed to Washington last week. Either answer is unsettling, to say the least.”