In his "Letter from a Birmingham City Jail," how does King feel about the Birmingham Police Department and how they act “'nonviolently’ in public”?

In his “Letter from a Birmingham City Jail,” Dr. Martin Luther King, Jr. says that we should not think highly of the Birmingham city police just because they have acted nonviolently during the recent protests.  Instead, he says, we should realize that their relative nonviolence has been a moral tactic deployed in support of an immoral cause.  We should realize, then, that it is only moral to be nonviolent if one’s goals are worthy.
Just before the passage you cite, King takes issue with the idea that the Birmingham police really are truly nonviolent.  He talks about how they treat black men and women very poorly.  He writes about police “dogs sinking their teeth into unarmed, nonviolent Negroes.”  He describes how the police “slap and kick old Negro men and young boys.”  He essentially says that they are not as nonviolent as his critics say they are.
However, King then goes on to admit that the police have been relatively nonviolent with regard to the recent protests.  He says that

the police have exercised a degree of discipline in handling the demonstrators. In this sense they have conducted themselves rather "nonviolently" in public.

Even so, he says, this does not make them worthy of praise.  The fact that they have acted nonviolently does not make them moral.  He points out that the police have acted nonviolently in order to “preserve the evil system of segregation.”  This, he says, makes their actions immoral even though they are nonviolent.  He argues that it is wrong “to use moral means to preserve immoral ends.”  Because the Birmingham police are doing this, King feels that they are immoral even if they are acting relatively “nonviolently” in public.
https://www.africa.upenn.edu/Articles_Gen/Letter_Birmingham.html