Then why don't these sabotages happen in US waters if it's so easy to get away with it?
Neither the EU countries, nor the US have enough ships to patrol or escort every civilian ship that happens to parse over an undersea cable.
Assume that the Russian shadow fleet starts targetting US cables. What would, could, the US do? The US government could easily retaliate by simply opening the US weapons depos to the Ukrainians. Target US infrastructure and Ukraine gets whatever it wants. The EU can't really do something similar.
Technically you're not wrong, if the EU navies where large enough, they could just escort every single civilian ship, but that's not realistic.
E.g. perhaps something along the lines of prison for captains who were 'derelict in duty', rewards to crews who grass and seizing ships that, by dragging anchors 'by accident', have proven themselves unseaworthy etc.
In contrast, the North and Baltic sea are a hodgepodge of individual nations' 12-mile zones, the Baltic Sea operates under an entirely different set of agreements that guarantee free passage, and it's not really deep so submarine infrastructure like cables can be hit by anything from anchors to divers with explosives.
