The reason why full face snorkels are not allowed in some places is because of safety concerns and the potential for carbon dioxide to buildup and also because of the difficulty in removing the full face snorkel in emergencies.
Other reasons for full face snorkels not being allowed are because of the risk of shallow breathing that can lead to hypoxia and also because the full face snorkel is really hard to breathe out the used air in your lungs and it's also even harder when in the water due to the water pressure.
Snorkels are not allowed in swimming pools because snorkels can be dangerous for swimmers that are inexperienced.
Snorkels when worn by inexperienced swimmers can increase the risk of shallow water blackout due to breath holding and can also obstruct a lifeguard's ability to quickly assess someone that is in distress.
The design of a snorkel can also make it difficult to clear the water from the tube if it fills up and potentially lead to water inhalation.
You can use snorkels in your private swimming pools but not public swimming pools.
Using a snorkel when swimming can cause people to hold their breath for extended periods and lead to shallow water blackout.
Shallow water blackout is a dangerous condition in which someone loses consciousness underwater even when they're are in shallow water.
Snorkels can also obscure your face and make it harder for lifeguards to monitor your condition and respond quickly to emergencies.
Some specialized swimming clubs and training programs may allow the use of snorkels under the supervision of a qualified swimming instructor.
And you often use specially designed snorkels to mitigate any safety concerns.