It's got nothing to do with the barrel being filled or not. All that is is extra work all around. It's about the firearm being rendered inoperable as a weapon. A missing trigger fulfills the exact same need as everything else I listed. Just filling the barrel doesn't. Maybe in ROTC they fill the barrel, but that's not the case for every firearm used for display throughout the US. All filling a barrel does is complicate rendering the firearm operable as originally designed. As the OP stated, TYPICALLY they are rendered inoperable. There's plenty of times when it's just left as is and nobody is any wiser. You actually think that every firearm in every parade is inoperable? Half the time it's nothing more than intent and a single cartridge away from being used as designed.

If it's common knowledge... Then why do you think the barrel is filled? Definitely plenty of ways simpler, cheaper, and significantly safer to render any firearm inoperable. Weld the action, remove the firing pin, or even render the sear inoperable. All filling a barrel does is turn a firearm into bomb since a cartridge can't vent to anywhere.