On my last flight to the US British Airways at first didn't allow my friend to board the plane, because they told us that her name is on some watchlist. An airline employee then phoned ~30 minutes with the US department of Homeland Security and asked my friend lots of questions - and then finally allowed her to board. Needless to say that we nearly missed the flight.

I can fully understand anyone who wants to avoid having a transfer at an US airport. Why risk any of those problems when you can just transfer via another country?