If you are offering them clean water all the time you are doing what you are supposed to. If it is there they will drink it. Yours have shown they know how. How long has this been going on?
Mine prefer to not drink from the clean water. If there is water pooled on the ground they seem to prefer that. If that water on the ground is clear they are likely to scratch and stomp in it until it gets muddy. Do they have another water source?
You still need to keep the water you offer clean. If it gets dirty it can become a breeding ground for disease. If mosquitoes can get to it you can be breeding mosquitoes. Even if it looks clean, I dump mine every two days and totally refill to stop mosquitoes.