For the general population who is healthy at that moment and needs to drink water there is no benefit to drinking packaged/bottled water over tap water. I think that is part of the reason they don't want it advertised on the bottled water.
The other aspect is for those who truly do become dehydrated something has seriously gone wrong. It may be due to serious illness (severe diarrhea/vomiting, cancer, severe infection, etc.) and the solution in that case is not to drink water - it will not be sufficient to handle the problem and may in some cases make you worse off.
The fact that drinking water rehydrates you is simple fact and there is no reason to bother putting that on bottled water. It is dumb advertising of an obvious health claim.