OK, here it is, maybe. I teach lifeguard training; pool, waterpark and waterfront training and prevention is the #1 goal of a lifeguard. Banning of the mask, snorkel and fins at a waterfront would be very similar to why floats and flotation devices are often banned at pools and waterfronts. For many, this equipment is somewhat of a crutch, or aid, in making the person more comfortable in the water. Keep in mind that most everyone on this board is very comfortable with mask, fins and snorkel but from a lifeguard's perspective, the average beachgoer is one who snorkels maybe a couple times a year. When that person is focused on the enjoyment of the underwater world, they tend to get "lost" and before you know it, they find themselves beyond their comfort level or in the midst of boat traffic. So the rationale is to eliminate the equipment that may lead to the situation. No, I don't agree with it but from a liability standpoint for some Parks and Rec agencies it would seem to make the most sense.
On a side note to an earlier post, if you ever have concerns about the lifeguards in your area, simply ask the guards what agency they were certified through and bring it to that agency's attention. While you may not be able to change the age limit, you can certainly demand a higher level of professionalism.