The conventional wisdom on College Confidential is that GPA and LSAT scores matter far more than undergraduate institutional prestige in LSAT admissions.
Here's a profile of the Yale Law School's entering Class of 2015: Entering Class Profile | Yale Law School
Notice that 68 undergraduate institutions are represented (including Albion College, Clemson, Gonzaga, Seattle University, and many state universities). The median GPA and median LSAT score are both very high.
It's true that at YLS, Harvard Law, and other famous, prestigious law schools, graduates of famous, prestigious colleges are represented heavily. I think the most plausible explanation is that those colleges, in their own admissions processes, cherry-pick highly motivated, capable students who are also good test-takers, and who may be very inclined to apply to famous, prestigious law schools.