I'm not using the word "overreacting" as a pejorative, but as a simple descriptive. The "overreaction" is simply doing more than the law requires of them. In this case, not taking americans as customers. The law does not require that of them, but they see it as too great a risk to their business.

I would also say that if the IRS is able to enforce their rules without war planes, then it is technically within their jurisdiction. The jurisdiction comes from a) legal jurisdiction within the USA and b) treaties that allow them to enforce laws beyond the USA.

Well technically it is not jurisdiction but treaties as well as leveraging economic clout and agreements.