The motivation is to keep (accidental) unlicensed users out of amateur radio service. For example, 2.4 GHz is both an unlicensed band usable under Part 15, and

The motivation is to keep (accidental) unlicensed users out of amateur radio service. For example, 2.4 GHz is both an unlicensed band usable under Part 15, and simultaneously an amateur radio band usable under Part 97, these are very different rules. Since amateur radio operators can use any public protocols (including 802.11) on the air, there is a risk of accidentally exchanging communication with devices owned by unlicensed operators (such as your neighbor's laptop), which violates radio regulations. Under this circumstance, some people believed that in the United States, encryption with a key published in a publicly accessible place is allowed (and there are still other people who says it's not allowed, but the debate has never been legally challenged until a high-profile abuse appears in the future, but so far there is none, so currently it is a de facto acceptable practice).