You are going a little bit on the wrong side here, I think.
Sure, Thailand is going to tax income as salary for all that do not work through a registered company. Exactly like in most countries. If you are employed and have a salary, you probably can´t deduct any loss in your home country either. Am I right?
Moreover, if you live and earn an income in Thailand, then you have chosen to live in a country that have a lower amount in exemption. That´s a choice. if you live in any other country, you will have to accept their rules for exemption as well. Same goes for allowances and other exemptions. I still think they will fix it, so that if any amount should be taxed in another country out of some reason, mostly a tax treaty will make a stop for double taxation.