Most universities group their grad students into their group rate for professors and other workers. Mizzou didn't do that. Most universities do not have a problem with grad health insurance, and actually have good benefits. Mizzou has terrible, terrible grad school benefits, and it's a real problem that will hopefully be addressed.
It's also not accurate to say that being a grad student is like having an internship. An internship you report to a group leader who leads your work, and that's it. As a grad student, you go to class, you teach, and (if you have a real grad school experience) you are responsible for your grad work. Having done both, having an internship is much less work than being a grad student, and should be compensated accordingly.