In the case of DNA, because it's shared with relatives and descendants who might be still alive. In Hitler's case, that might not be that much of an issue, but you were talking about dead people in general.

If your parents are dead, and thus they get DNA sampled, that information gained is good enough to positively identify DNA traces of all their children.

Remember how they caught the Golden State Killer? They put a DNA sample into the genetics website GEDmatch and found a few of his distant relatives. They then used publicly available family history records to construct a family tree that included all of these matches. That allowed them to narrow down the suspects to two people. One of them could be ruled out by DNA testing a close relative, which left the last one. They then took a DNA sample from his car, which was a match and that's how they got him.

Using that kind of stuff to catch killers is likely a good use of the technology, but there's quite a few nefarious things a state could do with a DNA database of all dead people.

You bring up a lot of good points.

Apparently there are some rights that exist beyond death. Currently HIPAA protection after death for medical privacy only extends fifty years.

So it does appear the argument is pretty moot.

Here is an interesting read on the matter.

https://aeon.co/essays/do-we-have-a-right-to-medical-privacy-after-we-are-dead

Because caring for our dead is a very human trait. In my state, a housing development was put on hold after the bones of indigenous people were found there and they had a connection to people claiming descent from them making the whole thing a family affair.

I may be very stupid about it and not know the normative, but what is the safest option for me is the following. No informed consent -> no research on any samples from the patient.

Does not matter how important your research is. I myself would like to be informed about that stuff. I may decide to donate my organs to research after I'm dead, but I have decided that.

Under that argument you could grind up the dead and use them for fertilizer. I guess if you're being 100% practical it makes sense, but humans have a certain sentimentality for their loved ones, dead or otherwise, and so don't tend to like it when you use the corpses in a way contrary to the wishes of the estate.