In my view indeterminism is simply used as a "God of the gaps".
There's no positive evidence of indeterminism. In fact, it's not even clear how indeterminism is compatible with scientific enquiry or human reason, since those assume determinism as their tenets.
What there is, is an absence of any clear mechanism that governs the workings at the "micro" level in a deterministic way. There isn't an absence of imagination of how things might work, but there's an absence of a fully worked-out theory and proof of it.
This absence of extant theory and proof is not the same as proof that there is no possible theory. All accepted science has passed through a stage where humanity didn't quite understand how things worked.
So this absence alone is not a good reason for thinking that things work differently at the micro level or that determinism ceases to apply.
We also shouldn't be surprised that progress is slow in this area, since so many so-called scientists have counselled themselves into a condition of learned helplessness, where many are not even theorising anymore about how the "micro" world may work in a deterministic way.
Effectively, those who reject determinism are actually anti-science - or at least, they are deadweight scholastics who retard the advance of science by typically heckling those who want to investigate or propose any new explanation of a deterministic kind, for no better reason than that no existing authority was aware of such a new deterministic explanation.
American science in particular has always been hostile to any developments in this area, because they have a dominant political ideology which lauds individualism and a semi-religious attachment to "free will".
They are hostile to explanations of human behaviour that don't localise to the individual, because the implication of explanations being non-local to an individual is that, when there is dysfunction in individual behaviour, you have to change the relations between individuals or alter something about the political environment.
It may seem tenuous, but for them, the idea that the brain relies on this "micro" world where things work differently and where effects don't arise from causes, is just what they need to keep talking about free will (and even divine intervention), and to keep the discussion off broader political change.
As for how they justify "macro determinism" from "micro indeterminism", the primary device seems to be that statistical analysis (and the observed statistical regularities) is asserted to be compatible with indeterminism.
It's not quite clear why this should be the case. By definition, we cannot analyse the indeterminate world in order to arrive at an explanation of why statistical regularities occur in the effects which originate from there. I suspect this is simply offered as a just-so explanation of how the world works, without any deeper justification for why an indeterminate world should be statistically regular.
To ask such a question is already to lapse back into the enquiring determinist mindset, of asking how an effect is justified by some cause.
The determinist, of course, would say that statistically regular effects arise from statistically regular causes, and since causes relate to effects in a consistent way, that is what answers the deeper "why" question of why the effects are regular.
We use statistics extensively to analyse deterministic situations - there's nothing about using statistics that implies indeterminism or requires indeterminism.
The application of statistics to QM is the sole case where statistics are said, by some, to be applied to indeterminism.