Thats because US corporate tax rates are ridiculous and so big corp plays games. Small corp of course are the real victims because they can't afford the army of lawyers and accountants to manage all the complex tax avoidance schemes so they get stuck with some of the highest corporate tax rates in the world.

If Apple pays a dividend, it has to pay taxes on it first. You are confusing that with their overall tax rate. It's low because they have a pile of cash that they aren't paying out