I've seen a lot of cats paw or scratch at their water bowls, even to the point of knocking the bowl over or into the sink - we have one of the water bowls next to the bathroom sink.
I don't think they are trying to spill the water. I think, instead, that they can't tell where the surface of the water is and they are pawing at it to make the water move so that they can see the surface of the water.
With a fountain, the water is always moving and the cat can see the surface of the water.
They don't want to stick their nose in the water and when the water in a bowl is stationary, not moving, they can't clearly see the surface - so they paw at it to get the water moving.
I doubt this is a "thought out" behavior, it's just something they do.
I've considered putting food coloring in the water in a white water bowl and watch to see if the cats still paw at the bowl but I've never gotten around to getting some food coloring to try it.
I'll add food coloring to the shopping list.