The sanctions go a little bit further than that though. Financial institutions are not merely blocked from accepting transactions from the IP region of North Korea, but actually from doing business with a whole list of companies and individuals with ties to NK. To comply with the sanctions regulation you would need to have a lookup service listing the owner of every wallet and for every transaction you validate, check if any of the wallets involved belongs to a sanctioned company or individual.
I don't think "but I didn't know that it was a NK wallet!" is going to hold up in court either. There are laws regulating the minimum Know-Your-Customer a money transmitting company needs to perform and when the regulators come knocking you need to be able to show your procedures for that and how they lead to compliance with the law.
The real question is whether courts will rule that POS validators are "money transmitters" in a legal sense. To me it's kinda obvious that they are, since without the action of the validators no money would get transmitted. No doubt there will be much water under the bridge before that gets settled though.