This action is being taken under pressure by the United States Government. For those that haven't been paying attention, last year legislation was passed that allows the IRS to levy a 51% tax on all assets over $600,000, including unrealized gains, for anyone attempting to expatriate from the United States.
EXCERPTS FROM THE TIMES:
Under pressure from federal authorities, the Swiss bank UBS is closing the hidden offshore accounts of its well-heeled American clients, potentially allowing their secrets to spill into the open.
The clients now face stark choices: they can cash their checks, and thereby alert the authorities to any potential wrongdoing, or not cash them, effectively losing their money. Or they can transfer the money to new banks, a procedure which, in the case of foreign banks, requires depositors of more than $10,000 to report the new account to the Treasury Department.
UPDATE: Some speculation is coming out via discussions at Free Republic (h/t: Mike Jezierski) and other places that this might have been the price of accepting TARP funds.