One would think that the ban was due to health or social harm reasons, however that is not the case. Unfortunately the ban was made for economic reasons to protect tax revenues.
During the 1940�s the Thai government was taxing shops and merchants that were selling opium. As these taxes started to increase, the price of opium was increasing in turn. Opium users started to look for alternatives and/or to break the increasingly more expensive habit altogether. Kratom use began to skyrocket as it was seen as an effective way to break the dependence on opium addiction. The economics pushed more people to seek out treatment.
Around this time Thailand was involved in the East Asian War of 1942, which increased the need to raise taxes. However, kratom as mentioned earlier a natural occurring plant was reducing the demand for opium, which was decreasing revenue for the Opium trade. Because the plant was growing wild it was impossible to regulate and tax it.
Therefore the Thai government had to suppress the competition for their opium revenue - making kratom illegal would serve just that purpose.
Police Major General Pin Amornwisaisoradej, a member of the House of Representatives from Lampang in a special meeting on 7 January 1943: �Taxes for opium are high while kratom is currently not being taxed. With the increase of those taxes, people are starting to use kratom instead and this has had a visible impact on our government�s income.� (1)The law was not found effective, since the tree is indigenous to the country. Today, kratom is scheduled in category 5 of the Narcotics Acts (1979), in the same category as cannabis and magic mushrooms (the least punitive category).
Source: Series on Legislative Reform of Drug Policies Nr . 13 Apr il 2011 "Kratom in Thailand" Decriminalisation and Community Control? By Pascal Tanguay
(1). Asnangkornchai, S. & Siriwong, A. (eds.) 2005. Kratom Plant in Thai Society: Culture, Behavior, Health, Science, Laws.