Because there's the expectation that regulated water systems are actually effective enough that we can use their output safely, while the licorice case is largely a case of self-induced tragedy.

Admittedly, there is a case to be made the licorice does have a political component in that we know it can be deadly, but do not mark it on the package. In my calculus though, water safety and fitness for use of municipal water output scores a lot higher on the public concern/public health meter than labelling one particular confectionary treat.

Both can reasonably be accommodated mind, but I'd be far more open to putting force of public mandate behind the one rather than the other.