For the vast majority of the degree, PhDs are not taking any classes (often, they are not even allowed to). They are just workers who either teach or research. The tuition is usually an accounting trick used by colleges/universities for tax and visa sponsorship purposes. Although faculty typically has to cover a PhD's tuition, factoring it in as pay is a little disingenuous.

If tuition was not waived, PhDs would not even apply. The way the PhD is set up in the US is a little byzantine. Other countries (Canada, continental Europe) do have normal employment contracts and benefits.