The critical issue here is that we don't know exactly what "US Person" means. Canada is in the throes of the same issue, with the US demanding access to banking records for any US Persons, and the scope of this is troubling.
US Persons includes US citizens, of course. But it includes folks who might be entitled to citizenship through birth or parentage, whether or not they are actual citizens. It would include anyone who has ever resided in the US. And the definition can be manipulated to mean whatever the US decides it to mean, down the road. It could eventually mean anyone who has visited the US or anyone who has a dollar-denominated bank account or basically anyone who they are interested in.
There is no burden of proof on the IRS to show that they are entitled to specific records. They can ask for anyone's records and claim "US Person" interest. Do you suppose they will not simply vacuum up everything?
And if there is any avenue for information to come to the US government, you know that the NSA will have it. And the DEA and all the rest.