Mosques try to spread sharia law in the United States. In Islam, sharia ("the Way" to God) theoretically governs every human act. But Muslims do not agree on what sharia says; there is no one sharia book of laws. Most mosques in America do not teach Islamic law for a simple reason: It's too complicated for the average believer and even for some imams.
And now, Spencer's answer:
The Saudi, Iranian and Sudanese authorities don't seem to have notable trouble sorting it out. In any case, the idea that Sharia is some complex and nebulous collection of arcana is a myth that is growing increasingly popular as Islamic supremacists try to advance Sharia in the U.S. In reality, "the four Sunni schools of Islamic law, Hanafi, Maliki, Shafi'i, and Hanbali, are identical in approximately 75 percent of their legal conclusions..." ('Umdat al-Salik, p. vii). Islamic law regarding the stoning of adulterers, the amputation of the hand for theft, the institutionalized discrimination against women and non-Muslims, is not hard to understand, and is not subject to notable disagreement.