I don't so much question the restraint used by the Capitol Police -- they were vastly outnumbered and literally unarmed. It was in the interest of their personal safety to not engage violently, though they obviously would have been justified in doing so. There would have been much more bloodshed though, on both sides, if they had, and they knew that.

However, it kinda looks like it was no accident that they were so outnumbered and unarmed (or at least backed up by armed law enforcement officers from other agencies). So, in the case of whichever higher-ups made the decision to pre-emptively stand down and open the gates, it seems highly likely that it was a politically motivated move. I think you'd have to have your head in the sand to think that it was any group from the left or made up of a different ethnic demographic that the response would have been so pathetically weak. I think that we would have seen a wall of National Guard surrounding the capitol with heavy firepower if it had been anyone other than Trump supporters.