Under FATCA, the Czech government has signed a treaty with the U.S. government where they agree to have financial institutions document whether or not U.S. citizens are account holders (and therefore, they pretty much have to document all account holders). So, if you do not fill out the W-9, the bank will have to close your account as you will not have provided sufficient evidence to document whether or not you are a U.S. citizen. Countries have been signing the treaties and starting to implement over the last couple of years - portions of the Czech treaty become effective in 2014.