First, the video you post does not "clearly show" that the police were letting protesters in.
Second, even if in some instances police were letting protesters through, why they were letting them through is a question that must be answered. The most pressing question, from the perspective of the interested public, is whether there were standing orders to let them do so or that there was some kind of conspiracy among the officers to disobey orders in order to do so.
So, do you see the disconnect? you have some (dubious) evidence that some group of police officers let people approach the capitol building, and on the other you have an accusation that the Capitol Police Department took the side of Trump and made a decision to endanger the lives of Congress.
This is not covering up the complicity of an institution. Neither is it shifting blame, because blame hasn't yet been assigned. It is simply a requirement that accusations be backed up by evidence.
I think it is fairly obvious that wrt BLM, the police officers responded the way they did because they the protests were directed at them. BLM threatens police legitimacy, police power, and demanded that those same police be held accountable for the wrongs they commit.
Explanation is not justification.