[anonetal]

I don't think this is that straightforward. One of the comments on the article said it nicely: graduate students are still being trained and are not very effective researchers, and they are getting a significant education in doing research even if they are not taking classes (whether it is "valuable" or not today is a different discussion).

Already the cost for a postdoc is comparable to that of a student (including tuition) and probably about twice that of a student excluding tuition, and their research output can easily be more than twice that of a student (on average). There could be adjustments to the model, but charging tuition to the students is not that outrageous when you consider what the students are learning. Tuition is also typically charged on the standard credit rate for classes (pretty low for public universities).