The September 11th attacks were carried out by piece of shit human beings; they were done on purpose by somebody else.
Hurricane Katrina was a force of nature, there's really no way to control it.
You can argue the levies were poorly constructed, but it's also safe to assume there are bridges in San Francisco that will fall down in an earthquake, or that resorts in Hawaii will perish in a tsunami, or hurricane could take an oddly north path and cause a storm surge so large it will wash out Manhattan. Is the solution to build 200' concrete walls around our shores? Rebuild every piece of infrastructure to ensure its integrity in an earthquake? While that'd be nice, it simply is unrealistic.
Your analogy of the poor levies by the U.S. COE is akin to blaming NASA for poor space monitoring should an asteroid plummet into downtown L.A.
Don't get me wrong, the tragedies brought upon by nature are super devastating, but our knowledge of science is continually evolving and we learn each day what mother nature can do. Not to go all Darwin, but sometimes I think death by nature is just part of planet maintenance. In contrast, death by hateful cowards is plain wrong, and I'd love for us to round up every last one of the terrorists and publicly hang them...by their cocks.