In Thailand writing a bad review of a hotel can land you in jail. Defamation can be a criminal charge, and include opinions deemed to be unfair.
British Airways can’t get away with putting its critics in prison under U.K. law but they’ve updated their terms and conditions to say that they can close the mileage accounts of members who “statements..reflects unfavourably on the reputation of British Airways or any aspect of its business, brands, products or services.”