The US Constitution guarantees, among other rights, freedom of speech and religion.
Contrast that to the European Court of Human Rights, which recently became the enforcer of sharia blasphemy laws. In a ruling, the court declared that defaming the Islamic prophet Mohammed was prohibited and exceeds the permissible limits of free speech in Europe.
This is in contrast to a ruling by the same court which allowed the insulting of Christianity in 2012.
They know that they will be murdered if they allow mocking Islam.