It's not immoral. Belief in free speech includes the freedom to choose which speech you want to support with your private resources. If anything, it's immoral to force private citizens to carry messages they disagree with. If the owners of CF are ideologically opposed to the ideas that 8chan represent, they should not be forced to be a vehicle for those ideas. Of course, booting people from the platform is bad for businesses, so it is wise for them to be as neutral as possible, but they have no moral obligation to support ideas they view as reprehensible. In fact, complicating normal business operations to take a stand against reprehensible ideologies is the moral thing to do.