However the problem is that the police considers this a good enough reason to jail someone working there legally after picking him up from his workplace.
I don’t think that’s fair.
There is literally zero evidence that he's working here (Thailand) legally. The article only says he works here, at a school.
That immigration police arrested him rather than "regular" police, suggests that his employment may not be above board.
Thai police can't speak English, so it wouldn't be out of the ordinary to ask the immigration police (which, by definition, deals with foreigners) to do it.
You can say the same about a good portion of the Thai immigration police.
But it’s the law in Thailand. And sometimes “those guys” believe that the laws in other countries don’t apply to them because they think it’s “not fair” and not the same as the laws in their countries.