It would be right for California to purchase the property via eminent domain. There is an existing mechanism, and I don't see why California doesn't do this rather than demanding a private citizen open his private property at a loss. This is like that bookstore in NYC, only Khosla is a Big Bad Billionaire so I see very different attitudes of "Oh, he can afford it." I'm not saying Khosla should keep the beach, but right is right; if California wants a public beach, she should re-purchase and pay for it.
Sure, if that was the law. But I don't think California law requires ca to purchase the land via eminent domain. The law is especially for this case where an easement is a defacto requirement when dealing with beach access. The legal process of emminent domain would be far too cumbersome for the many hundreds of miles of California beach line.