the knitting guild of america is a private corporation.
the Masters course is a good course, but its a product.
Its not accrediated by any organization.
It has the advantage of having qualified experts evaluate your swatches. (IT GETS YOU IN THE HABIT OF MAKING SWATCHES!)
You INVEST in the course, and you do the work.
You could teach your self, but for many of us.. unless we have a structure, (and the course does provide structure) and feed back--its hard to be motivated.
You can become a master knitter by just knitting.
by making swatch, by be realistic and reasonable about how good they really are..
but the course is impartial.. its honest. it is a standard..
Like any activity, you get out of it what you invest (in time, in practice, in money!)