
Totally pointless activity
Quite simply, under US law they can be served with a warrant for data (in some circumstances that isn't even needed) and they can then choose between coughing up a suitably large hairball of user information of close the shop - and in both cases they may be banned from talking about it.
(a status which, by the way, the UK government is quite keen to work towards as well if the most recent RIPA instalment is anything to judge by).
It Twatter wants to credible pretend it really wants protect users, its first move should be to move out of the US (there's more, but that first step is essential).