If they have one of the tank lids that include a filter, they might be concerned about the splashing that an air stone can cause. You can check this yourself by holding your hand above the bursting bubbles for a few minutes and noticing how wet your hand gets. The light fixture could also be affected if there is nothing between the water and the light fixture. Each bubble sprays a small amount of water a few inches as it bursts so anything sensitive to the moisture might be a problem for the tank manufacturer. The tank itself should not be affected at all but I suppose that doesn't mean a company won't delight in telling you the g'tee is void.
