We are not lawyers and by no means experts on Fair Housing legislation, nor can we explain why certain things are not protected by the Fair Housing Act. Protected classes are as follows: race, color, religion, sex, handicap, familial status or national origin. Some states might include "lawful sources of income" as a protected class, but that is not relevant to targeting certain income ranges on Facebook. Learn more about Facebook's advertising policy here: http://newsroom.fb.com/news/2017/02/improving-enforcement-and-promoting-diversity-updates-to-ads-policies-and-tools/
If you have any other concerns regarding Fair Housing and Facebook's Advertising Policies, we encourage you to reach out to Facebook or Respage directly.