I asked my stockbroker why part of my dividend payments were being witheld, despite the fact that I had filled in form W-8 declaring that I am not a US citizen. It turns out that there is a tax witholding on certain payments to foreign persons, including dividends. I am lucky that the UK has a treaty with the US meaning this is a mere 15% instead of 30%.
I imagine this highway robbery marginally reduces foreign investments. I wonder what interesting forms of taxation will surprise me next.