They have no choice: Israeli blockade has cut the provisions of chlorine to disinfect the water. Those who can't afford to buy bottled water have to drink that poison or die of dehydratation.
An affected mother described the effects:
Our kidneys and stomachs hurt, but I don't have means to buy mineral water. We are compelled to drink this water that is totally undrinkable.Original article at Rebelión (in Spanish).
In relation with this crime and similar ones in the occupied Golan Heights, the Scottish Palestine Solidarity Campaign is promoting an international boycott to Eden Springs (an Israeli bottled water multinational, that sells in many European countries).