That is EU law ..and it is to stop bottled water companies , who were trying to say that their bottled water ( identical to tap water ..but up to 1000 times more expensive ) could be labeled as a "health improvement product" or "medicine" that did not need a prescription..Sort of "anti dehydration supplement" to your daily diet..
If you eat and drink normally during the day..you don't normally ( unless you have certain "specific" health problems, or are taking a lot of excersise or in a particularly hot area or environmentn even need to drink water separately..normal daily water intake is fine at 2 litres per day ..what you get in fruit and veg' and food is included in that figure ) ..bottled water companies are trying to push their "health product" as extra to that..and "special"
And if you are severely dehydrated , you need special salts and compounds in the water that you are given to drink ..drinking plain water alone, without them, can kill you if you are severely dehydrated..
btw ..the EU didn't make a special law ( this was included in other consumer protection law about fraudulent advertising and labeling ) ..they said that the bottled water companies could not be included in the companies whose products actually do have "additional health benefits"..but the bottled water companies are trying to get support in the UK, by playing the EU dumb law card in the hope that the UK will give them a pass and let them say on their bottles that their water is better ( by insinuation ) for you than tap water in the UK..
It is not ..