Posting and closing beaches is pretty standard in California in response to sewage spills and water quality sampling yielding substandard results, though it’s usually enforced by local health departments instead of the state.
One issue is the public can get fatigued and disregard the warnings if popular beaches are closed on an ongoing basis, but at least they’re generally informed of the risks. Heal The Bay puts out a pretty good annual report card summarizing results for many beaches across the state if you’re interested in reading more.