It's a military base. The Space Force probably has something there that they want to keep hidden.
But what? A new space plane? A regular plane? Spy satellites? It's fun to speculate :)
Vandenberg is a Space Force Base, there's going to be times that the military has something on site they absolutely do not want filmed, and SpaceX has no choice but to comply.
It's not like Boca Chica, they don't own the Vandenberg launch site, they lease it from the government.
On top of the general space force argument, there's regulations about filming government sites and operations that get complicated quickly. From fueling to how it got to the pad to what security is on the pad to prevent a sniper from terminating the launch to the fact they don't want to pay for a space force guy to review every camera angle to make sure nothing secret is being shown, etc.
They've shown those steps before, though, and I can't imagine it has changed much. I think the hypothesis that there is some unrelated secret Space Force activity near the pad seems most likely.
A fully armed and operational vertical integration facility?
"Oh, I'm afraid this vertical integration facility will be quite operational when your friends arrive."
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