The difference is the advanced reporting function. As was mentioned, in a plane or a ship, there is no chance to disembark. For a bus coming across the border, if someone is obviously ill and is coughing, the screening officer, the customs official, would contact the medical officials. Even if the person did come into the country and sought medical attention, the provincial authorities are quite well versed in dealing with tuberculosis.
On April 18th, 2007. See this statement in context.