It is a safety issue.

When people submerge and hold their breath for a period of time, it can be difficult for the Life Guards to determine whether someone is in trouble or simply holding their breath. In larger pools or crowed pools, when someone is submerged and not moving for 30 seconds to a minute or more, the Life Guard must watch the individual closely to determine if the individual is really in trouble, thus potentially diverting their attention from a true emergency.

Pete