Falling on the ball

If it's really obvious and in a critical part of the match it would draw a yellow

By the letter of the law, yes. From the Laws of the Game, a caution (yellow card) is required when a player "deliberately and blatantly handles the ball to prevent an opponent gaining possession". The situation of a player falling onto and grabbing the ball, whether from a dive/flop, missed foul, or legal contact the player thinks is a foul is clearly deliberate, blatant, and preventing the opponent from gaining possession. Book 'em.

it's called a handball, and that makes me happy too, but I rarely see a card given. They blatantly grab the ball.