Currently a tourist can come to Thailand and receive a visa exempt stamp for free for 30/15days if travelling byair/overland. Or he can obtain a tourist visa ant an embassy or consulate outside Thailand which is good for 60 days.
If the same tourist once in Thailand decides to go to Cambodia, he buys a visa on arrival at $20, either at the border or a Cambodian airport.
So what is the difference here? I can see none. And it's taken 8 years to put this nonsensical scheme into practice, with no doubt dozens of meetings, lunches, hotels, expenses, travel to and fro between Cambodia and Thailand, all at taxpayers expense!