It may also be due to some UC research on the predictive value of unweighted versus weighted (uncapped) high school GPA on college GPA: http://files.eric.ed.gov/fulltext/ED492533.pdf
The weighted capped high school GPA may have been a response to give some incentive to take honors and AP courses, effectively becoming a penalty for those who do not but otherwise not really having that much of an effect for those who do (most applicants to UCs do take more than 8 semesters’ worth) and minimizing differences due to availability of large numbers of such honors and AP courses at some high schools but not others.