Why would Obama discriminate against UK in trade pact?

Guardian:

The Obama administration has warned British officials that if the UK leaves Europe it will exclude itself from a US-EU trade and investment partnership potentially worth hundreds of billions of pounds a year, and that it was very unlikely that Washington would make a separate deal with Britain.

The warning comes in the wake of David Cameron's visit to Washington, which was primarily intended as a joint promotion of the Transatlantic Trade and Investment Partnership (TTIP) with Barack Obama, which the prime minister said could bring £10bn a year to the UK alone, but which was overshadowed by a cabinet rebellion back in London.

The threat by Cameron's ministers to back a UK exit in a referendum on the EU raised doubts in Washington on whether Britain would still be part of the deal once it had been negotiated. More immediately, Obama administration officials were concerned that the uncertainty over Britain's future would further complicate what is already a hard sell in Congress, threatening a central pledge in the president's State of the Union address in February.
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Forcing teh Brits to stay in a bad deal that harms its economy and also keeps terrorist in the country make no sense.  It is in the US interest to have free trade agreements with both the UK and the EU.  We should negotiate a deal with both.