This doesn't actually stand to reason at all. Things do not exist in a binary state of "totally harmless" or "absolutely proven to be harmful". Typically the process for proving something harmful begins with weaker evidence, evidence accumulates, replications occur, longitudinal studies occur, correlational work gives way to causal work, and eventually regulators act to remove stuff. It's true that people shouldn't say "Chemicals are toxic so take my homeopathic cream and remove your fillings to avoid cancer"--I'm not saying this in support of quackery, and there's no shortage of quackery about artificial sweeteners out there--but something can absolutely be an area of concern without yet being proven. There's not one single valid approach to risk-aversion or avoidance. The precautionary principle should certainly be weighed against the costs of action, of course, but that's a judgment for different people to make differently.
Personally, it seems to me like the proposed effects of artificial sweeteners on satiety and appetite are plausible, and while I wouldn't go as far as saying "Artificial sweeteners are bad for you", what I would say is that people should be cautious about concluding that the proposed benefits of diet soda over regular soda (both in terms of caloric reduction and in terms of avoiding sugar in specific) are real. Personally, I don't drink soda, diet or full-calorie.