Fold for reuse. Soiled from food or beverages? Compost it. Lots of them good, clean condition? Post for free to local groups, like food pantries, for reuse on sites like FreeCycle, Craigslist, NextDoor, FB Buy Nothing, etc..
For information on Alameda County's Bag Law, click here, or on the California State Bag Law, click here. The City of Berkeley has a separate rule.