Societies subsidize people who live in flood plains because of agriculture, infrastructure and logistics. Essentially, we get far more OUT of people living in MOST flood plains than we put in with respect to subsidies.
Consider MOST of the coastal cities. We, obviously, get WAY more OUT of Houston, Galveston and New Orleans, for example, than we put in. Those three ports are enormously important to us. If they disappeared tomorrow, you would discover why they were important the hard way.
Similarly for people living in midwestern flood plains. We will typically get far more out of them than we put in. Try feeding the world, with currently known agricultural technology, while not using any land in flood plains and you'll see what I mean. Even if you could somehow manage to do it... and I don't think you could.... but even if you could, you would cause enormous damage to the ecological tapestry of the continent. You would have to move MATERIAL amounts of water and soil at a minimum. It's best to subsidize the farmers to do what they do and make sure they're not disturbed in their endeavors. And, yes, many of those farmers and agribusinesses will be in flood plains.
Now I'm not saying ALL flood plains in the midwest or on the coast are critical infrastructure. New York, Houston and New Orleans are far more critical to our survival than... say... Miami, Myrtle Beach or Malibu. So obviously, I'm not saying that ALL port cities or ALL midwestern flood plains are critical. But there are definitely some places in this nation that merit sizable subsidies to keep people living and working there. It's the nature of the beast when you're talking about engineering an empire.