Perhaps unauthorized is a better word than counterfeit. The manufacturing process for CPUs often yields less than ideal chips. Perhaps they don't hit the clock speed they're supposed to, or maybe they consume too much power. Those chips are supposed to be discarded, but they often find their way to the black market. Sometimes those chips aren't even failures. If a fab overproduces, they're not just going to give Apple the extra chips. These are the things Apple worries about, and they view it as far less likely to happen if those chips are made in the US.

I should also point out that the CPU isn't the only chip that TSMC makes for Apple. Apple wants to make sure they're getting a cut of every replacement part that gets sold. You can't even swap screens on two brand new iPhones without Apple giving you a hard time.