Well, there is a reasonable argument that cops, as a general group, do not regard white, right-wing, flag-wavers as threats. I think that if you Refaced all the folks charging the Capitol and replaced them with black and brown faces, there would have be a very different reaction.
And sure, things "went south" as soon as they were waved inside, because they were now inside. It's not like they came in to view the paintings and White House silver and suddenly decided to go nuts. They had a plan, they calmly walked inside, and they executed their plan.
How would you have done it if you were in their shoes?