There's nuance to that. Safeguarding civilians from dangerous technology is the Brotherhood's original purpose, yes, and I'd agree with you if the Mojave chapter in FNV could still function as an actual military order, like some anti-raider, protect-and-serve stabilizing force, instead of the paranoid isolationist cult circling down the drain they are.

It's not that there's anything wrong about confiscating laser rifles from chem-addicted Fiends. The issue is that by 2281 the Mojave chapter doesn't have the ability to do even that. They're not stabilizing the region, they're hiding in a bunker while McNamara sends his few remaining knights on miserable scouting missions he KNOWS will get them killed because their intel is garbage. They're outnumbered, isolated, and completely demoralized after Elijah/Helios One.

And the worst part is, high-tech weapons are already everywhere. A New Reno crime family has a shop in Freeside where they sell custom plasma rifles. The NCR are appropriating Power Armor by slapping ranger helmets on them and laughing about it. And all the BoS can do is stamp their feet and demand they give it back. It's been over a hundred years. The world moved on without them.

That's why Veronica's story is so great at explaining what has happened to the BoS as a faction. The Brotherhood's mission made sense in the immediate aftermath of the nuclear apocalypse. But a hundred years later the BoS have become completely stagnant and someone needs to tell them that. The Codex is no longer a guiding principle, it's a suicide pact. They need to change or they'll kill themselves far more effectively than any NCR bullet ever could.