Why do they do that!

It doesn't matter how often you put fresh water in their drinking bowls the both of them prefer to drink from the watering can. How spoilt are they, I keep it topped up from the water barrel for them.
When it was frozen they were licking at it like an ice lolly and I had to break the ice so they could get a drink. They have two water bowls, utility and upstairs in my bedroom, which are renewed each day. Maybe it's just the additives in the tap water because the only time they use them is during the night when they wake me up with their slurping!