Sharia law is the most important part of Islamic doctrine. Sharia is Islam; Islam is Sharia. Sharia includes law, but it also includes how to raise a family, theology, philosophy and every aspect of daily living. Sharia law includes pronouncements for both Muslims and non-Muslims (Kafirs). Sharia is a manual for a civilization.
Sharia does not allow free speech. It is forbidden to make a joke about Mohammed. Blasphemy is forbidden. The US is following Sharia when it allows the UN to determine that Muslim refugees come to America and not Christians.