from the article:
The bar owner was not arrested as archaeologists tried to establish whether certain of the antiquities had been declared to authorities, police said.
Under Greek law, all antiquities found in the country are state property. But sometimes individuals are allowed to retain artifacts after registering them with authorities.
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My view is that this is a ridiculous law, and that eating off ancient stuff would be kinda cool. Particularly on an island I've never heard of. :') It's not as if the restaurant owner spent afternoons skin-diving to find antiquities to sell out of the country.