As per usual with Thailand, I think the issues lies with lack of information. I regularly fly between pacific countries and on entry to those countries you are asked to sign a declaration in regard to specific items and as stated by others is a legal document.
I have flown into Thailand on many occasions but have never been provided with any written information or documentation advising on what I can or cannot bring into the country and advising for example volume of alcohol permitted or regulations pertaining to number of cigarettes (I do not smoke). Maybe this would also be an oppurtunity to advise travellers re vaping regulations.