The headline here is absurd and misleading. The lawsuit from the police state that they have the right to use necessary force to protect themselves. Nowhere in the legal brief is there a claim that they have "a constitutional right to use excessive force". Here is today's hint - based on the merits of the case they will prevail in this suit.
But "necessary force" is subjective. If we allow it, the determination of "necessary force" will become subjective. And before you know it, ANY force the ruling powers use and which you oppose will get you thrown in jail or worse. (Thanks go to Scat for the use of his boilerplate.)