Why should they allow the district's residents to enjoy a federal tax rate of, say, 15 percent, when they themselves may face rates close to 40 percent?
They should allow the residents to have a lower tax rate because most of them are poor. 
mnli hypothesis: They should allow the residents to have a lower tax rate because most of them are poor.  premise: Why should they allow the district's residents to enjoy a federal tax rate of, say, 15 percent, when they themselves may face rates close to 40 percent?