Not sure why people think that leather is somehow "free" given the cow is raised for meat. The value of the cow to the farmer includes all the components, including the leather. If people stop buying leather, then either the farmer has to put up the price of the meat to compensate, or raise less cattle to increase the price per cow through scarcity. Either way, slightly fewer people can afford to eat as much beef and less cows are reared, leading to less land, food and water being used and less methane produced.
It is one of Apple's values to move away from animal products: whether you like that or not is up to you and you are free to either stop buying Apple products in protest, or just buy a 3rd party leather case.