So lets say you are a manager for a normal 9-to-5 company, and head to Thailand on vacation. If you receive a phone call from work would that technically be illegal since you are doing your paid work from a thai beach?
I get that you are trying to push the example to the extreme, but it's hardly the same situation.
For a more realistic example: it is perfectly legal for a foreigner to come to Thailand on a business visa (without a work permit) for the purpose of meetings. There is even a specific 3 year (vs regular 1 year) business visa that is specifically designed for this situation (it cannot be used to get a work permit).
Wouldn't Thailand lose out by not allowing digital nomads to work there? What they contribute to the economy by staying there they would otherwise be forced to spend in a different country. So what is the law saying that you're not allowed to perform any "work" while you're in the country supposed to achieve, assuming you are not taking any jobs from locals?
The reality is more likely simply history - the Thai immigration system hasn't caught up with the 21st century.
The "traditional" situation is evident in other things too - for a marriage visa, there is a means test ($/month income or $$ in a bank account) - the amount required is vastly more for foreign men (marrying thai women) than it is for women (marrying thai men).
In terms of a theoretical "this is intentional" answer - I don't know, because I don't actually know what the average "digital nomad" would spend large amounts of money on - I'm living basically as a resident, I haven't left the country in 9 months, we have a car, imported our dog, etc.
For a more realistic example: it is perfectly legal for a foreigner to come to Thailand on a business visa (without a work permit) for the purpose of meetings. There is even a specific 3 year (vs regular 1 year) business visa that is specifically designed for this situation (it cannot be used to get a work permit).