Just because the security didn't use lethal weapons (except that one shot). And the reason they didn't use it is because there never was any real serious threat to anyone in power. So the fact that the mob got onto the building speaks only that the security preferred to let them in (fully understanding that everyone important is already evacuated) rather than killing them to prevent them from access.

Once in the building the mob quickly figured out that they don't know what to do next, as was expected.

> So the fact that the mob got onto the building speaks only that the security preferred to let them in (fully understanding that everyone important is already evacuated)

That’s...not true. The building was breached before members were evacuated. The breakthrough and shooting at the Speakers Lobby occurred, very shortly after that room was evacuated, and where the mob, if unchecked at the chokepoint through which they had just broken, would provide an imminent and overwhelming threat to both security personnel and innocents in the process of evacuating. Security lost control of the outer perimeter and did a collapsing bubble defense to maximize safety of the heavily outnumbered security personal and the ability to protect VIPs and other innocents who had not been evacuated.

Security didn't even use tear gas.