Guillen said what he said. He has that right. No governmental force is stopping him from saying it.
But any private group or organization has the right to criticize, condemn or silence an internal cog or piece that can and will harm its interests. That's why people can get banned from this website. The Marlins' interests include a large swath of Cuban fans, who clearly do not like Castro--which is why they are in Florida and not in Cuba.
Guillen always has and will have the right to say what he said. He can keep saying it as he is sitting at home on his couch, wondering why he is out of a job. He won't go to jail for it.