You can get away with adding some. I dose table salt now and then ( same as AQ salt - cheaper).... My snails are doing just fine. I haven't had to dose in high amounts though.
My understanding is that inverts do require iodine as a trace element in the tank. - I learned that here on AQ. You can buy additives that do the same without the salt.
My experience with this: I had a large red ramshorn and pond snail population when I used 2tsp per gallon of aquarium salt for a few weeks to treat ich. Unfortunately, the salt did not hurt my snail population.
In my opinion, if the salt was so lethal to all FW snails, there wouldn't be people having such a hard time getting rid of every trace of pesty snails in their FW tanks. A little salt would solve the problem without harming fish at all. Red ramshorns and pond snails are tough as nails, it seems.
However, it might depend what kind of snail. If I ever have to salt a tank again, I would definately move my mystery snails to another salt-free tank since I only have a total of 6 between all my tanks, rather than 60 or 600.
In conclusion, if you have any of the types of snails that are considered "pests" to most, chances are you don't have to cater to their needs for them to thrive.
there are only certain snails that tolerate marine salt, such as olive nerite snails, which breed in brackish waters. I'd also say table salt is a really bad move... if you need iodine, add iodine, but dont just make soup with kitchen ingredients.