> If the city / state is forcing the owner open the beach to the public, why does the guy have to pay liability insurance?
He doesn't. He chooses to, because insurance is more regular and predictable than the risk of liability.
> He should try to seek declaratory judgment that he's not voluntarily opening the land to the public, and therefore the state must indemnify him for any liability created by doing so.
That would likely be a waste of the money that he was paying to his lawyer to pursue such a suit; the law at issue was in place when he purchased the property, so the effect it had on the value of the property (including any associated liability risk) was part of what he chose to purchase.
> For that matter, if someone's not allowed to restrict access to their property, is it really their property at all, or is it the state's property?
"Property" is always a limited set of rights with respect to the subject of the property interest granted by the state.
> I'm no lawyer, but it looks like he's got a good case the state has de facto taken his property away from him, and he's owed compensation under the US Constitution.
Had he owned the property prior to the relevant laws being passed in the 1970s, there'd be some argument to be made that he had been subject to a taking. But he didn't buy the property until 2008, so, insofar as any public taking occurred with the public acccess provisions, they occurred prior to his taking ownership and were priced into the sale when the prior owner sold the property to Khosla. So, there's no reason even if the coastal laws were a regulatory taking, that Khosla would be entitled to any compensation. Khosla -- by virtue of the effect of the restriction on the market price of the land -- would be a beneficiary of the taking, not a victim.