[cauch]

If it's a precautionary principle, it should be called so. One should say "We don't know if smartphone is the cause, but by precaution, ...", which is clearly not the conclusion provided here.

You cannot say: I don't know if X is harmful, but by precautionary principle, X should be kept an eye on, so I can say unscientific conclusions such as "X is harmful" and everyone who complain that it is scientifically poor are incorrect.

Also, there is also a risk i precautionary principle: hiding the real cause while hurting the "good usage".