Yes, having more liberals working in policing is probably more inline with the goals of those liberals than not, but that's not the point of a boycott.
If the police can't get the help they need from mathematicians because of a large enough boycott then it gets them to the table and forces them to reform.
I'm not arguing whether or not this will work, or making a value judgement, I just see a lot of comments which don't seem to understand this point.