First and foremost, you need to understand that the whole first ammendment thing only applies to the government. So, in the context of free speach, the government is not allowed, with very few exceptions, to restrict topics of discussion. Private individuals, however, are not restricted in this regard and are free to limit speach as they see fit, limited only by the extent of their authority. Thus, I am perfectly free to tell anyone that within my house, certain topics, words, or modes of expression are not allowed and will result in punitive action. Since corporatations are under the law private individuals, they are also free to restrict speach.