It’s not a matter of political correctness; it’s a matter of preventing the military from appearing partisan by any measure. Showing patriotism with American flags isn’t a problem and never has been. Showing a political campaign symbol is, based on current existing regulations.
I know.. it sounds anachronistic, but the military’s conformity, discipline and impartiality are among the traits that keep it the best in the world.
Little, in anything, will happen to these guys in this instance; they’ll simply be reminded what’s authorized as a display on military vehicles and what isn’t.
Maybe at the moment, active service members can still get by with waving a US flag in a public place off base.
But when enough time goes by with nonstop yammering into everyone's brains about how the US flag is a "Divisive Symbol" with racist undertones second only to the Confederate flag, who knows? The Left has long defined it as a Political Symbol that is to be reviled, and I'll bet you that well over 1/3 of the country now believes this, just due to the constant repetition over decades.
So yeah, the military members, and all the rest of us, should enjoy their/our current right to proudly display the colors. Because that right will not last forever, as it will be seen as a symbol of resistance to the Federal Govt. At least with Trump's election, the schedule has been set back by many years.