Islam does not believe to each his own.
Islam is an aggressive proselytizing religion that puts even the hardcore city harvest christian to shame. If Islam takes over the institutions, be at a national or street level, it would do its best to restrict or even snuff out non-Islamic elements. You can see the sphere of freedom for non-Muslims shrinking in Malaysia. It's even worse in places like the Middle East or Islamic parts of Africa.
If a local m&d quits Islam, all his muslim friends and family will disown him. Getting family and friends to exert peer pressure is a good way to police Muslims and ensure they stay with the flock. Most local Chinks, Hindus and Christians won't do that to their friend or family if they leave the religion for Islam.
