When foreigners own US bank account/certificate of deposit, are their interests subject to US tax withholding?

Answer 1

If a non-resident alien of the U.S. has interest income from a bank account in the U.S. they are exempt from U.S. on that income. This only applies to a personal bank account.
If the bank account is connected to a U.S. business then the income will be taxable in the U.S. For example, if the non-resident owns a rental property in the U.S. and excess rents are put into a bank account that earns interest such interest is taxable on the same return as the rents. If the excess rents are shown as being distributed to the individual and put into a personal savings account then the interest earned would not be taxable.

Answer 2

Yes. The bank will be required by law to withhold a flat 30%, and then you have to file a US nonresident return. If you are a citizen of a country that has a tax treaty with the US, you may be subject to a tax rate on interest income that is lower than the 30% and then you would be able to get the overage refunded.