I'm not sure this is the case. According to people who live on Roatan, only the West End area of Roatan is a national park. When it was established the airport people were overly diligent and confiscated all shells, no matter where or how they were collected (even purchased ones); but that has been relaxed. As long as you aren't collecting in the park, you should be able to collect and bring your shells home.
Utila Island, Honduras
I know the area is now protected and thus prohibited of any collection