The Defamation and libel laws in Thailand are complicated. They are not the same as in other jurisdictions and foreigners can easily make the mistake of incorrectly assuming that exceptions to rules and justifications can apply to forgive violations of the law.
Desmond Hughes has written and had published an article on this topic in Window on Phuket, the Image Asia publication which focuses on matter of importance to Phuket expatriates; Phuketians and visitors alike.