Who could be so amazed by sand that they'd smuggle 40 keys away? Money had to be the motive and in that case, they definitely knew something was wrong with taking it.
Google led me to:
"Why are the sands of the Sinis Area so precious? The reason is because there are no more sediment reserves that can be deposited on the beach, on the other hand the sea erodes the beach year after year, thus gradually reducing the size of the beach. That’s why it is absolutely necessary not to take the sand from the beaches of Sinis"
,