We know that parallel construction has, at various times and in various places, been heavily used by some law enforcement elements. We don’t need to know anything about the software used by law enforcement to know that, and to suspect that this may be happening now. Systems such as those provided by Palantir are just a more sophisticated way that tainted evidence could be used.

It seems reasonable that, given a reasonable suspicion that such evidence is used, that we should fear and want to prevent more widespread and systematic use of that illegal evidence.

No, this is more specific than just using a DB. If they use Postgres to handle HR then it's irrelevant. This is specific to using large quantities of potentially tainted information and then not disclosing the use of that information. Which is more or less the definition of parallel construction.

Further the problem with parallel construction is not that it hides tainted evidence it's that the courts can't decide that the evidence was tainted. Even if all the information was fine the technique inherently creates problems.