Bah. The "tackle from behind" rule is being totally misapplied nowadays.
The rule was introduced - rightly - as a direct result of Italia '90. What would happen in those days was players would tackle opponents directly from behind, catching them around the achilles or lower calf, which by definition meant they had no chance of getting the ball. This type of foul became a poison in the game because it allowed a defender to easily break up the attacking team's play - for example, it was effective against any attacker who was trying to hold up the ball during a counter attack. It happened a dozen times a game and was so negative and ruined so many games in that WC that it left the authorities with no option but to change the rule.
The rule was never meant to cover every tackle that originated from "behind" an opponant, only those tackles that went through the back of the player's legs with zero chance of getting the ball. Running alongside, but just behind the player, and tackling from there you can get the ball and is not what the rule was introduced to police. The rule has drifted and drifted and is now nonsense as routinely applied. Note that it has been successful in stopping the blatant fouls described in para 2 above.