The reason is simple: it's a EULA violation. /share, which is only obtainable by paid means, gives a team an advantage over the opponent.
Picture this scenario: an unranked player has Giant unlocked by no coins (because he spent all his coins unlocking the Giant). And a ranked player has 6000 coins but doesn't have Giant unlocked. So the ranked player /shares all his coins to the unranked player, then the unranked player sends in his Giant and essentially wins the game. You get the general idea, do you?
Picture this scenario: an unranked player has Giant unlocked by no coins (because he spent all his coins unlocking the Giant). And a ranked player has 6000 coins but doesn't have Giant unlocked. So the ranked player /shares all his coins to the unranked player, then the unranked player sends in his Giant and essentially wins the game. You get the general idea, do you?