If a cop lets you off with a warning, he is under no legal obligation to give you one again.
This is a little different than a traffic ticket.
I don't know if this woman and her son were aware of the deal struck with King Farouk, (I would presume on something like this they would do some level of research before saying, "Look what I found.") But the disparity between splitting proceeds with him and seizing coins from them is astounding. Also, they were clearly aware that their father sold some of the coins without charges being filed.