For one, that this data is improperly anonymized would make it an easy avenue for malicious nation-state actors to use to track/analyze/destabilize the population. If I am a government with an interest in freaking out the US public, I could quite easily de-anonymize sensitive datasets and begin using them for wide-scale harassment, identity theft, etc. on an automated basis.
The lowering of the bar makes it easier for Johnny Troublemaker to start harassing people based on their PII as well. Instead of paying for the data, just download some datasets and run a Julia notebook against them. Maybe not much changes for the targeted stalking case, but now you can cast a wide net when looking for someone to mess with.
The number of Johnny Troublemakers who are randomly spraying hate based on PII is about the same as the rate of people throwing rocks off highway overpasses onto cars below. It's simply not a significant enough problem to be worth worrying about.