When you fill out required tax forms you had to send them to a Financial Crimes Enforcement Network even though you never did anything wrong. Wouldn't you feel wrongly accused of being a criminal?
Your are under threat of penalties from $5,000, $10,000 or half your life savings if you make a mistake in this or that form.
Imagine that you can't save for your retirement.
Imagine that you can't make any investments-
Imagine no bank, investment house or insurance company will do business with you.
Imagine your bank
Tells you to pay up your mortgage in 30 days.
Closes your accounts.
Rescinds your charge card
Rescinds your debit card
Won't give you a loan for a car, your business or any reason.
Imagine your company
Can't deposit your paycheck because you have no bank account
Denies you promotion because you are American and the post requires that you can have signatory power on its accounts.
Have to give up your role on a board of a social organization because it will show up on a bank account and even if you are the only American among hundred of members all their financial info will be reported to the US.
This is what is facing 8.7 million Americans including those that have only had the misfortune to be born in the US (and only spent a short time there) or to a US parent living outside the US. If you couldn't live with those fears, than you will understand why expats are upset.