I'll sort of recant the outside market forces have "no impact" mainly because I don't like talking in absolutes. It's possible that some of the decisions some law schools have made regarding with their tuition have been related to legal hiring. The graphs have alot of holes to try to posit that tuition hikes have been a direct response to legal salaries. The two big missing components are how long the blank check lending practices have been around and the impact of US News (since 1987, right around the time tuition started it's precipitous rise). As to the latter, students and admissions offices make decisions based on a ranking system that rewards a school for spending more money. In terms of blank check lending, I'm not 100% sure how long that's been around. Grad Plus loans have only existed since the mid 2000's and I'm not totally sure how the PLUS loans before them worked. One thing I am certain of is that in the 70's you didn't get a blank check from the govt., or if you did alot of people were unaware that it was available.
The bottom line is that if you will lend someone money without underwriting it, someone out there is going to take it pretty much regardless of what the product they're spending it on is.