Save your blood and accept the simple fact that "the police" are a actually a set of more-or-less independent small government departments who get to beat people up and get away with it. And then arrest the beaten up person putting them in jail indefinitely before trial, and creating an immediate huge financial and social burden on that person. Mostly the public is unaware of this because the frog has boiled in fits and starts, the last big temp increases happened because of 9/11 under Bush and Obama and the distro of military gear to PDs. Tell me: what about this situation implies "self-restraint" is a key virtue of PDs?
I'll tell you what the problem is. It's the justice system itself. It's extremely slow and expensive. Defense lawyers have perverse incentives -- delays make money. Prosecution have perverse incentives -- they just want easy wins. The courts have perverse incentives -- they don't want to decide cases. So you get a system rooted in all-powerful patrolmen who effectively serve as judge, jury and executioner, with the rest of it serving as a willing, very highly paid window dressing. So why would a PD ever say no to getting any legal advantage?
The solution must be to rethink justice from top to bottom, such that excesses like this are punished meaningfully. I'm uncharacteristically optimistic about the possibility, mainly because of computers and the internet. Justice is about making sound decisions informed by reality after applying agreed upon rules (laws), and it requires a judge/jury. Computers can help with each of these elements in ways barely explored by modern-day courts. They can help with information gathering and jury selection/operation. If we wanted, we could have same day trials of higher quality than the months or years long things we have today.
For people aware of these problems, the only solutions they hear involve fixing the justice with new legislation or by electing the right sheriff, mayor, police chief, or if a higher level of government steps in (like the FBI or federal courts). Or even worse that odious pablum "just vote" - if you've been served shit for dinner it won't taste better if you push it 'round your plate.
(If someone with infinite money (and with open-minded connections with judges in various courts, and supportive politicians) wants to fund a serious effort in this area, please reach out to me, because this need is critical.)