Batter throws bat

[Eastshire]

Carelessly is not the same as unintentionally or the same as illegally. You could intentionally release the bat in an effort to hit the pitch but do so carelessly and hit the catcher instead.

If you're going to throw the bat at the ball, it should be at the ball and away from the catcher (or really where the catcher should reasonable be expected to be).

If it's thrown carelessly, even if it's a legal attempt to hit the ball, I don't see why you wouldn't issue the warning the same way you would when the player releases the bat on the follow through and sends it flying.