In no way is this legal advice but upon my own research it appears that general awareness of this objectionable data is not enough for legal prosecution of validators.
Prosecutors would recognize the lack of intent. Since blockchain users are not able to delete this data due to the immutability of blockchains, the intent appears to matter.
Section 512 of the Digital Millennium Copyright Act provides protection for intermediaries who are just doing their jobs. Section 230 of the Communications Act also provides protection for other kinds of content, which is why Twitter and Facebook weren’t shut down when one of their millions of users posts defamatory statements, child pornography. It appears from a legal stance, those who blockchain nodes are performing similar functions to internet service providers or web hosts.
Other examples include peer to peer file sharing cases in the 2000’s, companies like Kazaa and Grokster were shut down since their services were primarily used for copyright infringement. In contrast, the majority of content on the blockchain is for financial transactions. Even if pornography is recorded on the blockchain, it would be hard to argue that its the intended use. Same goes for counterfeit items, like when Tiffany sued Ebay for listing counterfeit items, Ebay had general knowledge of the trademark infringement but wasn’t held responsible.
Same would go for POA, Bitcoin and other blockchains in my opinion. These are all valid concerns and interesting topics to discuss regarding innovative technologies and how they apply to law.