Saint Helena has import tariffs like other colonies do. They used to have preferential rates for commonwealth countries but I'm not sure if that still exists. I guess my response would be they have import duties for the same reasons other colonies do. Why wouldn't they tax imports?

edit: found an updated tariff document https://www.sainthelena.gov.sh/wp-content/uploads/2022/08/ST... . I don't see anything special for commonwealth countries so it's probably a thing of the past.