The second job is more difficult to do effectively (with the potential that some of the protesters were armed, any armed response could have escalated into an actual battle), and was almost certainly compromised by the absence of the personnel needed to evacuate and secure the legitimate occupants of the complex (I don't see any numbers at a quick glance, but almost certainly thousands of people on what was a normal workday until 2:30PM).
Not to mention the inexplicable absence of last summer's no-badge secret police in guarding this protest, and an unwillingness of the executive branch to provide requested national guard allocations to support the Capitol Police.
That does not mean that they should have ever allowed protesters to attack the Capitol building while Congress was in session.
It's also important that the BLM protests were not about wanting to overturn the results of the election and install their preferred candidates. The cause that is at least proclaimed by a protest movement matters, it is not just a detail when considering a valid response to it. And even so, any police officers who fought BLM protesters to keep them from attacking democratic institutions, especially those in session, were fully justified.