Charging PhD students tuition after they are finished taking classes (which typically accounts for the majority of their years) is racket that allows the university to double dip into research grants. The first dip comes from 50% "overhead" they skim off every grant.
Further, in my experience, few professors actually do any research nor write papers. The vast majority of this work is done by PhD students and to a lesser extent post docs. Thus, PhD students are the linchpin that holds the entire "do research to publish papers to write grants to do more research" cycle together. If all PhD went on strike, the research output of a university would dry to a trickle, which would make obtaining further grants difficult to impossible, thereby depriving the university of that luscious 50% overhead.
Said another way, PhD student are net value producers for the university. Forcing PhD students to enroll in "thesis hours" that cost 45k per year is simply a cleverly worded scam to pilfer even more off the grants.