A judge in CA could just do that, or even not charge any bail. They have a lot of discretion. However, some judges may decline to offer low (or no) bail. That's where the "no cash bail" elected officials come in. Some DAs, for example, make it policy not to ask for cash bail (and obviously the defense attorneys or defendants will also ask for no cash bail).

Judges can't do that because you can't just wholesale eliminate cash bail without other changes as you can't just release everyone. The new laws are necessary to introduce alternatives as part of that elimination.

For example, the law in California would give judges risk scores to switch to a risk based bail system.

Risk scoring has been problematic after AI/expert risk scoring systems produced politically unpalatable results.

It is difficult to see a productive way out of this dilemma since "correcting" the input data is a priori impossible to do in an objective way, and skewing the algorithms will almost certainly produce unintended effects (e.g. a violent criminal being underscored, released, then they victimize innocents again).

Whether the risk score is discriminatory on protected classes can be easily measured. (After implementing a risk score system, is the fraction of bail jumpers equivalent across groups?) I assume that kind of measurement is what prompted the overhaul of cash bail.

But unless the system is either completely perfect (i.e., its predictions are never wrong), or completely worthless because it ignores all input data and always gives the same risk score, then there will always be some measurement that says it's discriminatory on protected classes: https://arxiv.org/abs/1707.01195

The paper you linked to describes three different definitions of fairness that don't agree. I gave a single definition of fairness that is self-consistent.

Yes, if everyone agreed that your definition of fairness were the right one (which I personally do) then there'd be no problem. But in practice there will be people who want to use other ones (in some cases because they want to be able to say it's discriminatory).