The Sharia punishment for Muslims who drop their faith is death and the confiscation of their property.
The reason why is curious.
The Muslim Brotherhood's Abdul Qader 'Oudah explains it in "Criminal Law of Islam, Volume III": "If this offence is taken lightly, the collective system of Islam may collapse. The severe punishment laid down for it aims at the total elimination of apostasy on the one hand and warning and preventing others from committing it."
Opposition to Islam must be silenced at all costs in case the ideology can't withstand free thought.