Living under Sharia law is central to Islam. A Muslim who rejects Sharia law is an apostate. Under Islam, the penalty for apostasy is death.
Even if such a person is not killed, he or she would be barred from entering a mosque, and fellow Muslims would be barred from associating with him or doing business with him
And if a Catholic person violates various Church laws, he or she would be barred from Communion. If an Amish person violates various religious or community laws, he or she would be shunned. What's the difference?
The article makes clear that criminal acts would not be covered by Sharia law. As long as that distinction is maintained, I don't see a problem. If a Muslim is physically harmed or killed for rejecting Sharia (or for leaving the religion altogether), then that must be prosecuted by the government as assault or murder. And if the physical harm is either sanctioned or instigated by the Sharia court, then everyone involved is an accomplice.