Mackey's case is an interesting one. I think that there are a couple of issues in play here. The first is that the issue of free speech is one that citizens can enjoy. However, I think that in a business or a corporation, those rights are not always as guaranteed. Consider how the Supreme Court, and lower courts, have ruled in this situation:
Fundamentally, an at-will employee in the USA can be terminated at any time, and for any reason – or no reason at all – and the courts will not intervene to protect the ex-employee from allegedly unfair treatment by the employer. Courts have repeatedly recognized that "any reason" includes a "morally wrong" or "morally reprehensible" reason.
The leader of an organization has an even greater responsibility to recognize the power of their words. In Mackey's case, his argument against global warming was made in a consistent manner with his libertarian philosophy. The board of directors or management, in general, has to assess two elements. The first is whether or not Mackey's words or ideas is consistent with the corporation image that Whole Foods wishes to present. The second is whether or not Mackey's words should have been uttered in the first place. Mackey has had challenges with whether or not such element should be articulated with blogging and similar forms of expression. I think that all company executives and employees should grasp that their thoughts are not necessarily protected by free speech in terms of their capacities as representatives of such organizations. While they have free speech as citizens and individuals, when it comes to organizations and corporations, they are obliged to speak in line with those organizations.