I suppose they are banning it for the same reason some have banned holocaust denial. Denying such events is dangerous and the type of people who do are NOT people you want in your country if you can avoid it...
I know what you mean. Only a fool would deny that something happened that did, or deny that something that is so isn’t. If certain UN functionaries had their way they would ban the criticism of Islam.
If a governmental functionary told you it was unlawful to refer to Islam as anything other then a peaceful passive religion would you believe it? I sure as he’ll wouldn’t.