Forgive my ignorance, but unless they found that a person had a receipt, how could the CBP possibly determine if something was purchased on the trip (unless it was really obvious, like T shirts that said Caribbean on them or bottles of rum) or if you brought it from home? Like, what if someone brought a lot of expensive jewelry from home on the cruise (Not that it's a very good idea, due to risk of loss or theft, but some people might do it...), and the CBPs try to accuse them of purchasing it on the trip? And how could they prove it?