The rules notwithstanding, the point is that if someone is sick on board an airplane, it is difficult to get medical attention before landing, and therefore it makes sense to have advance reporting so the necessary arrangements can be made at the port of entry. If someone is on a bus, certainly there are lots of options in terms of getting the appropriate medical attention prior to the person arriving at the land border.
On April 18th, 2007. See this statement in context.