I think that @anon145 has it right and we should not get hung up on the gross GPA score when comparing students to students from other high schools. For in state students, the state universities know every high school in their state and those students are competing against their peers in their high school, not with students across the state. They typically know about how many from each high school they can/will accept. It is not a black and white cutoff, but if you were to look at any individual high school for last 2-4 years, I bet you would see a fairly consistent cutoff of GPA/Class Rank.
We could debate whether GPA/grades are an appropriate indicator of likely success, but if you are a university with highly competitive admissions, why would you take a risk on someone who is highly intelligent but has not demonstrated achieving up to that potential when there are sufficient candidates demonstrating high achievements in the classroom, even with lower standardized test scores?