Well, some table salt has iodine, but not all does, especially if you start looking at sea salt or kosher salt. In fact, I need to buy some more sea salt, all the flavor, half the sodium, and I just ran out.
Common table salts contain iodine and anti-caking agents which can kill your fish. Of course there are table salts that don't have iodine or additives, like Kosher or rock salt (the latter I use), which are safe. It's not the salt, as all salt is basically the same, it's the additives they use in it that make it unsafe.
As for plants, small doses won't kill them but in moderate does for long periods of time will. I just didn't want to take the chance as I had delicate plants and wasn't sure on their tolerance for salt as it varies.
I may go pick up some aquarium salt and let it sit in a cup of water to disolve before putting it into my tank. I have 7-8 scaleless fish and I don't want to harm them or cause them discomfort.