California Code, Business and Professions Code - BPC § 13550

No motor vehicle fuel producer or distributor shall compel or unduly or unreasonably influence any retail dealer to participate in the giveaway or offer to give away free of charge any item of value, including trading stamps or any kind of merchandise or goods, whether or not the giveaway is conditional upon the purchase of motor vehicle fuels or lubricants.  The decision to participate in those giveaways shall be solely that of the retail dealer.  Nothing in this section shall prohibit a retail dealer from entering into an agreement to participate in any giveaway program.