We're stuck between having to do timid actions and full NATO escalation. This feels like constant creep.
No one ever seems to want to discuss what to do about the bear going around poking everyone else.
Also, once you are 12 miles offshore, technically you are in international waters and thus cannot be stopped by any Navy except your own unless there is UN Sanctions. If NATO Countries decided to violate that, it obviously opens up massive can of worms that could impact worldwide trade.
But IIRC the TLDR is it has to do with indemnities and putting a vessel/person up for prosecution after the fact. And it doesn't apply if cable damaged while trying to prevent injury, which RU can always claim.
More broadly I think you're correct on paper... RU damaging subsea infra is under UNCLOS is technically punishable, but after the fact. And they're not going to lol pay damages to countries that sanction them. NATO kinetically trying to prevent RU damaging subsea infra (especially in highseas), in lieu of formal UN policing mission against such acts, is closer to act of war.
Of course, that would also be true of NATO doing so as part of a broader collective defense operation reported to the Security Council, directed against Russia and explicitly aimed at rolling back the Russian (UNGA-condemned) aggression in Ukraine under Article 51 of the UN Charter.
International law can be selectively applied for different party according to different scenarios (relative to different geopolitical power). NATO triggering art5 (self defense) won't make it valid / feasible to trigger at parallel UN art51. RU using UN art51 to target UKR a soveign territory, is also going to be different than NATO / or NATO country using art51 to do whatever they want on non-soverign / international high seas. All of which is to say while international law doesn't matter much to the motivated, not everyone is powerful enough to normalized/destablize with impunity. NATO might, but not without RU security council (trumps UNGA) approval, of course NATO can supercede from UN Charter framework which IIRC that NATO explicitly states they operate within. But then we have NATO going independant of UN, which goes back barrels of worms.