At the moment I do not think we should all have the same salary because in every area the cost of living is different. If you live in San Francisco or NYC you cannot pay rent with 2000$, while in some countries rent would be only 250$ a month. Same with health care, food, etc.
In the first place, salaries shouldn't be based on location, but the offline cooperate companies have to pay according to the cost of living in your environment... Subsequently, the online market stopped to care about such a concept of payment. Thus, every online worker started earning as a result of the level of impact they add to the company or industry.
For example, freelancers on same level of skill like Op mentioned earn similar salaries regardless of their nation. In a nutshell, with bitcoin being the medium of payment the location of a writer in USA won't be enough reason to earn higher than another writer in Africa. What now determines their salary is the skill and impact they've got to offer for the brand.