I agree that it makes no sense for Russia to do it when they can just turn it off.

There would be diplomatic hell for the US to pay if it was done without permission from the German government. Going solo on this would be very risky.

The German government will have political hell to pay with its citizens if they find out their government did something to Jack the price of energy even further. Good way to get your political party ostracized if the people find out.

So, who else would be motivated to do?