I agree that people are really unlikely to be deported for this, but I'm not sure that countries would treat you favorably if you walked up to immigration and said "I'm here to work in your country on a tourist visa, but don't worry because I'm just working remotely for a foreign company."
I think a big part of this is that it's basically impossible to find digital nomads. How would a country know if you're in a hotel room or coffee shop working on your laptop? If you don't tell anyone what you're doing, they aren't going to know.
Digital nomads also don't trigger the same "taking a job that a local could have" fear that drives a lot of immigration policies. As you note, you're just a person in that country spending money. However, you're also probably not paying taxes. I think there's a bit of a tension there. On the one hand, you're bringing in foreign money. On the other hand, you're using services just as a resident would and not paying all the taxes (you are likely paying some taxes like sales/VAT).
Ultimately, I think a big part is that countries haven't figured out what to think of digital nomads. Heck, companies haven't figured out how to feel about them. Companies still haven't settled on whether remote workers should be paid less if they move to a cheaper location.