The California Supreme Court in Avila v. Citrus Community College District (Cal. Supreme Ct. – April 6, 2006) explores the "beanball," "brushback," and "chin music" … and primary assumption of risk doctrine in California: 

Being intentionally hit is likewise an inherent risk of the sport, so accepted by custom that a pitch intentionally thrown at a batter has its own terminology:  “brushback,” “beanball,” “chin music.” In turn, those pitchers notorious for throwing at hitters are “headhunters.” Pitchers intentionally throw at batters to disrupt a batter’s timing or back him away from home plate, to retaliate after a teammate has been hit, or to punish a batter for having hit a home run. (See, e.g., Kahn, The Head Game (2000) pp. 205-239.) Some of the most respected baseball managers and pitchers have openly discussed the fundamental place throwing at batters has in their sport. In George Will’s study of the game, Men at Work, onetime Oakland Athletics and current St. Louis Cardinals manager Tony La Russa details the strategic importance of ordering selective intentional throwing at opposing batters, principally to retaliate for one’s own players being hit. (Will, Men at Work (1990) pp. 61-64.)…"

See also California Appellate Report at http://calapp.blogspot.com/.