Apple didn't patent a "resistance", they implemented a control-chip in their headphones starting from the iPod Shuffle in ~2009, and patented that chip. Purpos

Apple didn't patent a "resistance", they implemented a control-chip in their headphones starting from the iPod Shuffle in ~2009, and patented that chip. Purpose was to ensure a revenue-share from headphones so that every accessory-maker who wanted in-line controls had to pay a license to Apple to use the control-chip. (the control-chip was meanwhile reverse-engineered and its functionality is now integrated in non-licensed headphones as well)