Participante anónimo 641 multi entry tourist visa reportedly (I don't know and I don't care, either) doesn't allow to work remotely from Thailand. The DTV does, so I can say out loud both to Thais and to people in my country that I'm legally working from Thailand.
but the DTV was created for that: for medical treatment, for courses, etc. It's not the same as using a student visa to work. Of course, if anyone is using the DTV to work, it should be also revoked, but the examples you mention about dental treatments and cooking courses are indeed within the scope of what the DTV is for.
Remember the DTV is good for 5 years. If what you intend is to do a short course of muay thai or Thai cuisine for a couple of months, you can get the regular tourist visa, which allows you to stay for up to 90 (60+30) days and has way less stringent requirements.
Sounds good to me. No country wants people without money to come and live here. Your income doesn't need to be from a job, tho. Own business, renting properties you own, etc., everything counts.
Somebody doesn't know to read the rules. Don't get me wrong, it doesn't make me happy that this happened to you, but the rules are in writing, so you can't claim not knowing them.