While I agree with you about the difference between "insure" and "ensure," that's not a universally accepted grammar rule. I think the may even be in the minority; The New York Times, for example, uses "insure" for all instances.
It does? I have a 5th Ed. New York Times Manual of Style and Usage right here and it clearly distinguishes between the two. Under the entries for both ensure (p 120) and insure (p 170) it gives examples of the other.
Well, I guess that's what I get for believing my first Google hit for "insure vs. ensure." Nonetheless, I see a lot of highly literate people using "insure" for all cases.
