It's a similar thing to custom skins. Using a custom skin is technically altering part of the game, which is clearly outlined as being bannable, however Rito allows it because it does not harm anyone. They can reverse their decision at any time.
Chat macros do not hurt anyone under certain situations (not used for spamming/clogging the chat so no one can use it), and so they are currently "allowed" only because Rito hasn't banned anyone for it.
Macros that assist in playing are the other side of the coin for macros. They alter the game to give you an artificial advantage, which is clearly outlined as against the rules in the agreement you sign every once in a while when logging in. You can and will get banned for it.