It's a bit of a difference, but many of them are which is exacerbating the problems is that everyone who can in some of these places are leaving for other jobs.
But the difference between the EMT scenario is that there is that there is a potential that by getting involved you may not go home to see your family that night. In addition if you get involved and something goes wrong, you're going to be the next police officer on the news for "police brutality" and the rest of your life you'll forever be in fear of your life even if you did everything right. Furthermore the job of the police is not to protect citizens or stop crimes the supreme court has ruled as much, the police are an investigative force.
With the DA angle makes it even worse, because you spend the time and effort to arrest somebody and then they are back on the street a day later, what's the point in the first place. To put it other terms let's imagine for a second every day your boss took all the code you wrote and deleted it all, deleted all the backups and everything else, are you going to continue putting your best effort into coding, are you going to make sure the code compiles and the unit tests past? No you're going to do just enough to not get fired.
We can argue it shouldn't be that way but that is the reality of the situation, it turns out when you demonize a large group of people simply because they do their job, and criticize all of them as inhuman monsters and tell people that every single one of them is a bad person they start to lose some motivation.