What does Thailand gains by scaring those people? After all they spend a lot of their wage money there. What's the downside?
On a tourist visa, or "non-working" business visa, it is commonly illegal to do work even if for a foreign company and for a foreign market in many, maybe most, countries.
The visas don't tend to take into consideration the nationality of the company you work for, or the notional audience, because it'd make it trivial to work around (set up a "foreign" company owned by some trust to hire you, set up another foreign company to buy what you build, and have said companies contract with a local company), instead they regulate the type of activity you can engage in.
(e.g. I used to
You can attend meetings, sign contracts, make investments etc.
Non-B visas are also needed if you want to get a work permit. You can't get a work permit whilst on a tourist visa.
Starting your own company in order to get a work permit requires 2,000,000 baht ($62,000) of registered capital and social security payments 4 Thai employees. There is also reasonably sized list of businesses you cannot start. You also can't own your businesses outright unless it's approved by the Board of Investment.
It's very difficult to do business there legally.
The policy is fairly straightforward, but letting people do "digital nomad" work while trying to stop those taking jobs in a country isn't so easy.
A bunch of countries in SE Asia seem to have economic development people in favor of attracting these people to their country. But coordinating with the immigration enforcement authorities isn't very well developed (given the systems in place this isn't an easy problem it will likely be messy for awhile).
And if long term visitors are seen as negative (often for acting out and making asses of themselves) this can lead to crackdowns. A few jerks are very noticeable, so a large community of good visitors can be seen as a negative based on just a few bad actors.
The U.S. and China both offer an expatriate exemption (first $100k in income for the US) and foreign tax credits against U.S./China taxes for local country income taxes paid.
As a short term tourist, you're a valuable resource. As a long term resident, you may very quickly become a net drain if you pay your tax somewhere else.
And for most people, if they live and work in Thailand most of the time, they will either not pay tax in their home country, or will pay tax proportional to the amount of time they live there, on the expectation that someone who is resident outside the country most of the time will not expend much local resources.
Laws have dealt with this kind of scenario for a very long time since it's been an issue in border areas pretty much "forever" that people try all kinds of creative methods to cut their tax costs. There's nothing particularly new here with the internet other than magnitude.
The biggest notable exception that can cause expensive double taxation is the US, since the IRS likes to get their hands on US citizens income regardless where they live, but even in that case it's ameliorated substantially for most countries via double taxation treaties.
The tax argument only holds water if your income is earned in the country you are living in. That money came from a system that is supported by government and shared infrastructure and services. So yes, there is a strong legal and economic argument saying if you earn money from Thailand you should pay taxes in Thailand. The key here being earning money in/from Thailand.
But if you are visiting Thailand, but earning money in the US and paying taxes in the US, it's pretty difficult to say that you are a net drain.
You absolutely are equivalent economically to a tourist as far as your impact on the economy is concerned. As a non-local citizen, you won't have access to state health care (if it's provided) or anything else tax paying citizens have access to. And your absence from the country would be a net negative value since you will be paying $0 into the local economy instead of room and board, internet connectivity, and general consumption, which all can be taxed with a VAT.
Edit: Also, I promise I'm not trying to be an internet jerk :). But I'm not sure what you mean by laws have had to deal with this exact scenario for a long time.
They haven't.
Generally speaking, the common case is for people to evade taxes by leaving where you earn money and claiming to be a citizen of some other country with more lenient tax laws. Which is the total inverse of this scenario.
In the Netherlands, for example, >50% of income tax is received from the 10% highest earners. So it's not unreasonable to expect a high earner to contribute accordingly.
EDIT: Heavy downvoting, but this is how things are in most Western democracies, like it or not.
If you spend $5 a day while earning $5000 a month, it's better to have one person who really loves the country and wants to stay paying income taxes, rather than 100 staying and paying VAT.
The typical government member in my experience tends to think in terms of "people will just pay more" rather than "they will go somewhere else".
EDIT: Moreover: how should a government discriminate? If you are working for a company in Norway you are probably earning better than someone working for a company in laos. If both of you are residing in vietnam, should I expect the latter to spend more in VAT than 50% of the vietnamese college graduates ?
That's a pretty stupid viewpoint in this context, considering I've already demonstrated my willingness to leave and go somewhere cheaper (Canada -> Vietnam). You'd think they'd rather have something from me than nothing.
As longer term expats tend to say "TIT" (aka this is Thailand). Don't expect it to be consistent or make sense to a Western mind.
[Email in my profile.]