Since when wasn't it? Do you suggest we put law school admissions up for public vote?
I suggest that if the state is going to run a law school, the legislature should dictate objective admissions criteria only, such as tests scores and grade point average. If the bureaucrats want, they can keep statistics on the law school grades received by admittees from different undergraduate schools and discount each school's GPA's accordingly. However, if you go beyond that to allow soft criteria such as how the applicant did on an interview, they are going to shy away from conservative applicants. Kennedy alludes to the disdain for GOP applicants in his otherwise weak dissent.
The problem with that is that even the makers of standardized tests don't recommend overemphasizing the test scores and as you allude to, GPAs from different schools are directly compatible.