"You can't kick the ball out of his hand."

England kick off to start the second half of their match with Scotland. Jonny Wilkinson kicks off high. England knock the ball back and a tackle/ruck forms. The ball comes back quickly to scrumhalf Danny Care who picks up the ball. He has it in his grasp when the Scottish lock Jim Hamilton puts out a long leg with a big boot and knocks the ball from Care's grasp.

The referee says that it is wrong to kick the ball out of an opponent's hands in that way.

Sez who?

This is where it is stated:

Law 15.5 (j) When a tackled player reaches out to ground the ball on or over the goal line to score a try, an opponent may pull the ball from the players possession, but must not kick the ball.
Sanction: Penalty Kick

This is reiterated in Law 22 on In-goal.

This is not near in-goal and Care is not trying to score a try.

But why is the law there for in-goal? It's there because of the potential danger of kicking the ball out of a player's hand. Presumably that danger exists anywhere in the field, and if there is danger a referee is required to act.