There are many legitimate answers, but I'll leave it to others to give their intelligent answers. Your interesting question will attract more non-Muslims for further discussion and thus reopen a new floodgate of discussion on Islam.
However, I will briefly tell you one of the many answers - that is to warn or educate others about the 'road of no return' if one embraces Islam.
It is illogical not to allow non-Muslims to discuss Islamic matters. Like Christianity, Islam is a missionary religion with much zeal to propagate its belief.
If I was being told of the philosophy of Islam by a Muslim and I had a lot of issues in doubt - don't I have the right to further question and discuss it before I make a decision to embrace Islam?
Don't my non-Muslim friends and relatives have the right to inform me of the 'inconveniences' of Islamic practices, which we may not be aware of? Should I not further discuss controversial issues and make a research on comparative religions, seeking help from both Muslims and non-Muslims friends?
Or is Coklat 2004 trying to tell non-Muslims just keep quiet and listen?
When Islamic laws encroach into the daily lives of non-Muslims (like the canteen and dog ownership issues), should non-Muslims just remain silent and automatically lose their rights to discuss and explain their plights, just because the discussion concerns Islamic law?
Truth is always open to criticism. And truth will always prevail. If God is with Coklat 2004 they need not fear. History has taught us that many regimes, emperors and religious fanatics have used draconian laws to restrict others from freely sharing their thoughts.
God is not so evil that you are not allowed to ask 'why'.