Ultimately I'll point the fault at the district attorney, as they are the ones who decide what is prosecuted and what is not.
They take unsubstantiated allegations from hertz and put people to jail, but at the same time don't lift a finger to prosecute the clearly guilty corporations for fraud. So yes, while mistakes are made all around, the ultimate guilty party is the district attorney.
For it to be the DA's fault, lots of other things have failed first. So I don't really see how the DA is that relevant. If devs failed by making software that doesn't work in a way that the company using it cannot keep track of their inventory in a way that makes it look like their customers have not returned items in a way that looks like it has been stolen so that they can make a report to the police who cannot properly investigate which results in someone being arrested but never charged does not make any sense for a DA to receive any blame in this situation at all. (jeebus that must be the longest sentence I've ever typed).
Because only they are the one with the power to actually act on this and pursue charges. They are supposed to validate the facts before proceeding, but here they just take the word of hertz without any evidence (since hertz's systems are messed up as you say so they don't have any idea what's where) and run with it, accusing innocent people.
The DA is the one with the power to tell hertz to produce credible evidence or go pound sand. But they don't.