The airline I work for does not allow us to serve potable water unless from the boilers for hot drinks. All cold water we serve comes straight from bottled spring or mineral water. I know another airline such as Qantas use potable water for the majority of all their services.
There could be several reasons for that, serving cold water from the tanks requires that you have good quality water in there, which means regular cleaning of the tanks and taking care of where you take in water. Some airports provide water that (though safe) tastes worse than the potable water in other airports. On the other hand, bulk is significantly cheaper than bottled, but supplying only bottled reduces the maintenance required for the potable water system. (Finances may balance out differently domestic vs. intercontinental.)