The moral question in this case isn't "Should you be able to unplug the violinist?" but "Should the violinist unplug themselves?".

As an adult who is profiting from a crime committed against you, they are morally obligated to return you to your original state that existed before the crime was committed (even though it would return them to their original state, which was illness and near-term death).

For a child in the womb, though, they do not have a pre-existing state (under the philosophical premise that conception is the starting point of a unique human), and most conceptions are not the result of crimes (although I can understand someone drawing a bright moral line between different pregnancies based on that factor).