Yikes, let's break apart your claims.
Why am I arguing this? Your initial claim, that protestors are trying to burn down the federal courthouse, is totally unsubstantiated and smells like bullshit. It's worth arguing against because it paints protestors as violent agitators.
The horror of arson does not excuse extrajudicial precrime tactics. In fact, it is reasonably well-argued that no crime can excuse extrajudicial tactics, simply because committing crimes in the case of seeking justice for crimes will lead to never-ending criminal acts.
By what evidence are you not convinced that these buildings are fire-resistant? The positive circumstantial evidence is that, from both an encyclopedic source and firsthand hearsay, the building is composed of quarried stone, steel, concrete, marble, and bronze. The negative circumstantial evidence that you've brought up, that the office-interior decor may ignite and that the building may burn out from the inside, is not just speculative, but doesn't meet the original criterion: The building would not burn down just because it has fire damage inside. Even worse, once again your unfamiliarity with the building hurts, because you don't know about the marbled entryways and stonework walls, which don't have enough wood in them to sustain fire damage.
And then you bring up 9/11. What a red herring. What a non sequitur.
Finally, you failed to address a sibling point: That after the Oklahoma City bombing in the 90s, the Feds stepped up their stance on defending federal public buildings like courthouses. We are not really exaggerating the level to which these buildings have been secured. Again, this is something that you may have to visit and see for yourself to grok.
Please watch less Fox News, or whatever it is that is rotting your arguments in this way.