They are enforcing this part of the FHA (emphasis mine):

>(c) To make, print, or publish, or cause to be made, printed, or published any notice, statement, or advertisement, with respect to the sale or rental of a dwelling that indicates any preference, limitation, or discrimination based on race, color, religion, sex, handicap, familial status, or national origin, or an intention to make any such preference, limitation, or discrimination.

If people actually did place housing discrimination ads they'd also be in violation of the law.

(In general, laws are usually written in a way to prevent profiting off of illegal behavior)