I agree this is annoying, I believe this feature was originally added as a way to easily empty water buckets. It doesn't make sense when you're emptying a bucket into a single source block sure, but when you think of it as emptying it into an ocean it makes more sense. Either way I believe this is a bug not a feature, since it has been in the game for so long and because it is much more convenient to empty a water bucket into other water instead of placing water on the ground and replacing it with a block.
hunderslash
I agree this is annoying, I believe this feature was originally added as a way to easily empty water buckets. It doesn't make sense when you're emptying a bucket into a single source block sure, but when you think of it as emptying it into an ocean it makes more sense. Either way I believe this is a bug not a feature, since it has been in the game for so long and because it is much more convenient to empty a water bucket into other water instead of placing water on the ground and replacing it with a block.