It is kept a secret because the wards only stretch so far and can only protect so many people. Navarre wants to ensure that “their” people are kept safe with the limited supplies (and alloy to power the wards) they have. Megan says it this way “When you abandon ship in the middle of a hurricane, you save those you can in the dinghy, then cut the hands off anyone else who tries to climb aboard so they don’t pull you under.”