I think there's another aspect to it...
One of the exemptions here in CA on the statewide mask order refers to dine-in restaurants.
- Persons who are seated at a restaurant or other establishment that offers food or beverage service, while they are eating or drinking, provided that they are able to maintain a distance of at least six feet away from persons who are not members of the same household or residence.
The idea isn't that if you and your friends all want to go out to brunch together, that you should do so and not wear masks. Restaurants/bars are not intended to be opened so that large parties of people not from the same household will congregate together and just happen to be 6' apart from other large parties of people not from the same household.
This is one of the keys to a beach closure. Young people don't go to the beach alone, and they DEFINITELY don't go with their families--Ew!--they go with their friends. Their friends who live in different households.
So maybe that group of 6 friends stay 6' away from another group of 6 friends, but again you're mixing households and by doing so drastically increasing the likelihood of spread.
Now, it's entirely possible--probable, even--that many of those groups of young people will still form and have July 4 BBQs at one of their houses instead of going to the beach.
So will it have much of a practical effect? Maybe not. But the issue isn't dad, mom, and their two toddlers going to the beach and staying away from other beachgoers, it's people who are NOT of the same household being close together on the beach, even if they stay apart from other beachgoing groups.