What about zero-nicotine e-liquid(s)?

According to Federal Food, Drug and Cosmetic Act, tobacco products are defined as “any product made or derived from tobacco" and also includes "any component, part, or accessory..." such as electronic cigarettes or electronic nicotine delivery systems (ENDS). Simply put, as long as a zero-nicotine e-liquid can be used in anything deemed as a tobacco product by the FDA, it is also considered a tobacco product and hence all restrictions apply.