Because the EU has a very different reaction pattern and overall resources. It was never a question about downing ships that are sabotaging cables.
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.
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.