Not only was it Germany's chance to relieve itself from fighting in multiple lines, but it was also a great opportunity to launch a large-scale counterattack against the British and French forces on the Western Front.
As for Britain and France, they currently did not have enough power to interfere with the revolution in Russia. Moreover, the current Russian revolution has not affected the monarchy, but merely established a constitutional monarchy similar to Britain, which was not unacceptable.
If Russians, like the French Revolution in history, were to overthrow the monarchy and execute Nicholas II, they would certainly be held accountable by Britain, France and other monarchical countries after the war.
The European royal family could allow European monarchs to lose their power, but they would not sit idly by and let European monarchs be executed by revolutionaries.