Because what they're asking is "What if we could fundamentally change existence?" That's what it would take to perform these reforms without the existence of free will. Because people wouldn't have the choice to do whatever.

So while we would have to accept that not only are criminals incapable of doing anything different, but also the police are incapable of doing anything different, and the people hiring them, etc. Turtles all the way down.

Nothing changes.

This is an oversimplification/misreading of the philosophical ideas behind the notion that free will might not exist.

Proponents of the 'no free will' camp generally understand this to mean that we are not the conscious authors of our thoughts, and that thoughts are the product of one's lifetime of experiences and the result of those experience on one's specific biology.

In such a model of the world, the police would absolutely be capable of change, since knowledge is a potent change agent.

We already society is capable of this kind of change by looking at the history of slavery. A growing understanding of the value of each human life made the abolition of slavery a moral imperative for similar reasons. Such a cultural shift didn't happen overnight, though.

And if free will absolutely does not exist and we're just the results of the sum of our experiences, then it's also inevitable that the abolition of slavery was going to happen because that's the sum of the total.

Free will is binary. You can't have just a little of it. And if the argument is that a lot of what we do is unconscious, then that's not arguing against free will. That's arguing against something else.