Where do they draw the line on this?? It is getting to the point of being ridiculous with what could be used "As a weapon". Your keys, your belt with the buckle, your eyeglasses or sunglasses (sharpen the ear piece to a point). A bar of soap in a sock or a roll of coins in a sock- All can be used as a weapon, but they let those items on the plane in a carry-on.
I believe they are just using that as an excuse for confiscating the item for themselves. You should have taken the conch shell and walked over to the trash can, and broken it into pieces and threw it away. Then reply "OK, it's safe now".
It's sad, but just another example of how someone with a minimum wage job is using their "power" to control you. They were probably bullied when they were growing up, and are trying to get back at everyone now.