Not that he cares or that I'm trying to incite anything. It's just important that we not forget this important point.
The reason you would rule that way is because California can't simply go in and change your property rights, at least not without compensating you and using eminent domain. That has been established for a long time. So for the same reason they cannot just decide to give access to the minerals under your house to Chevron.
So it isn't "squatters rights" at all, rather it is property holder rights. If this is upheld it could have some interesting repercussions on water rights in the Central Valley if I remember, some of those were parts of Mexican land grants as well.
That's both news to me and a wincing moment of "wow, California politics really do come down to water issues 90% of the time".
His oft repeated quote was "when politicians agree on something and it stays out of the news, it's usually not in the public interest".
People like to make fun of Las Vegas as a city that wouldn't exist without imported water but most of So Cal wouldn't either. The bad guy in the film Chinatown is loosely based on William Mulholland, the chief engineer for the Los Angeles Department of Water and Power.
California law requires the property owner to give up rights. The eminent-domain laws require the state to compensate property owners when forcing them to give up rights.
Conclusion: The property owner must allow beach access to the public, and the state must pay the property owner for allowing this.
EDIT: The comment by gojomo would seem to explain this.
I like the idea of working out a 'patch' to the treaty, although I don't think I've heard of that ever happening in practice, new treaties yes, but patching an old one not so much.
If the US promised, in the 1848 Treaty of Guadalupe Hidalgo, to honor in perpetuity the property grants of Mexico in the conquered/ceded territories, maybe that promise still applies. And if the Supreme Court already ruled (in 1859!) that California law can't alter those rights, just as state law is limited in other international and interstate affairs... well, it's an interesting case. Would love to see Volokh Conspiracy or Popehat discuss it.
Khosla may yet want to cede this claim, even if it's legally legitimate, to not appear like a robber baron buying up eccentric anti-democratic privileges that were created by ancient wars.
Update: And another thought... maybe if the US and California now view the coast – and the water-rights angle that ChuckMcM mentions – as requiring a new approach, they can negotiate a treaty-patch with Mexico? :)
To make things even weirder, there are a bunch of cottages/houses on the property that are being leased to occupants (this was done by the previous owner). From what I understand, they have a few more years on those leases before they can be evicted (I am very rough on these details). So if you know someone in those units, they can hook you up with access to the beach (I think).
Bascially, it's a crappy situation for everyone. I get that Khosla wants some private beach front property to dump his money into. And really, the previous owner should have set up an easement (easements are the primary reason Malibu is still publicly accessible), although he probably would have got a much lower sales price. And of course, if Khosla gets his way, the public will be getting screwed over pretty bad. With the growing population, beach/coastal access is getting harder and harder (try going to the beach on a warm, sunny weekend, it can be a parking/traffic nightmare). Reducing the number of coastal access points is just going to make things worse, and it is going to continue to have an impact on the next generation of Californians.
For the sake of the community and historic use he should restore the access rights that were there. It is the right thing...but, hey, it's his property.
 This Constitution, and the Laws of the United States which shall be made in pursuance thereof; and all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land; and the judges in every state shall be bound thereby, anything in the constitution or laws of any state to the contrary notwithstanding.
It's not to say that it doesn't haunt our government, European Court Of Human Rights is just a treaty after all, and it fundamentally changed our criminal procedure a few years ago, but we integrated this treaty inside our Constitution, and allowed it to hit us on top of our laws.
"Martin's Beach was previously a popular family beach and surf spot before Khosla purchased the property adjacent to the beach and blocked access."