My guess (and clearly just a guess) is that the recent drowning of the pre-teen at pop century - and the related negative publicity (it made lead story on cnn.com at one point) - may have prompted an overall review of safety/adult supervision policies.
Maybe a good guess. From the article linked by the OP:
The new policy will take effect March 23 at all U.S. Walt Disney World and Disneyland resorts and parks.
Sounds like the policy might be applicable at the resort pools.