Normally, snails will not eat aquarium plants because of toxins the plants produce to prevent fish and snails from eating them. Because the defenses terrestrial plants employ such as tough cellulose walls, thorns, needles, ect... are impossible underwater, nearly every aquatic plant is not edible to fish or snails.
With the exception of highly specialized plant-eating animals, like Silver Dollars, most aquarium plants are fairly safe from the animals in the tank. It's possible that Apple snails are capable of eating aquatic plants, or perhaps your Aponogeton is unhealthy.
If the plant is unhealthy it usually becomes vulnerable to snails and fish. This is a valuble cleanup service in the planted aquarium, because most of the unhealthy tissue would end up as dead tissue, which would need to be removed mechanically.
