Yeah, but even when you process food, you still have to list all the ingredients - those don't magically vanish after processing, y'know?
It may be that the water the smoke bubbles through frees the smoke of ingredients which make you cough - I mean, there has to be a reason to let it bubble through water in the first place.I don't know about that... ever tried a hooka? Way less harsh than a cigarette, even using pure tobacco. Of course, American law enforcement's war on personal liberty and self-ownership of our bodies makes them rare in these parts ("it's a [Marijuana] bong, seize and destroy it" ), but they seem to provide a counter-example to your assertion.
edit: According to Wiki, the water cools and humidifies the smoke. That would explain it.