Not always true. Some things may be tax free in your resident country (e.g an ISA in the UK) but pay normal tax rates in the US. Since credits from each income

Not always true. Some things may be tax free in your resident country (e.g an ISA in the UK) but pay normal tax rates in the US. Since credits from each income category are separate, you may not have enought credit in the 'bank interest' category to cover what the US demands (I'm simplifying but that's essentially the issue). It may be a small amount but it's still owed tax and I'd imagine it's quite a common situation.