My assumption would be if they were to see someone with a foreign address come and try to cash out 1000 BTC in USD (or even 10BTC currently) on one of the exchanges, then they would want to know where those bitcoins came from. It would be able to combat another form of layering (with the normal process of placement, layering, and integration). So "at border crossings" here would probably mean any bitcoin exchange operating in the US that deals with foreign entities.