Its to do with the fact that, if anything does goes pear shaped ,US investors cannot claim for associated investment losses where the trading occurred on exchanges outside of the USA.
This has already been established with regard to BP and another company who name escapes me at present.
Here is one link to the BP issue :
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http://www.ft.com/cms/s/0/5e25bf2e-8de3-11df-9153-00144feab49a.html#ixzz1BK7ctuzbLawyers aiming to sue BP on its shareholders’ behalf are scrambling to deal with the fallout from a court ruling that could bar the bulk of investors from seeking damages in the US.
Attorneys specialising in so-called class action lawsuits said they were looking at ways round a US Supreme Court judgment that appears to prevent claims by the almost three-quarters of BP investors who bought their shares on foreign stock exchanges.
http://www.ft.com/cms/s/0/5e25bf2e-8de3-11df-9153-00144feab49a.html#axzz1BK77WqCWThere is an even more significant finding from a foreign courst but that's the one which eludes me at present