The parent's rather obvious point is that some police officers are bad, but police in general aren't behaving this way. If you disagree, then fine, but at least disagree with the parent and not some straw man of your own making.
To the comment of "Yes, the person who brings the baton down on your head is an oppressor." - bringing up 'well other police are not bad' is just itself a weird strawman.
It's not a straw man so much as a clarification, because the original assertion is ambiguous about whether we're talking literally about a single person or if "person bringing the baton down on your head" is a metaphor for police in general (especially given the surrounding context of talking about the police in a general, plural sense, and the fact that no one disputes that no one disputes that individual bad police officers should be held responsible).