I'm not a civil rights attorney and this is not legal advice, yada yada. You're referring to the exemption in 42 U.S.C. § 3603(b)(2), which says: "Nothing in section 804 of this title (other than subsection (c)) shall apply to . . . rooms or units in dwellings containing living quarters occupied or intended to be occupied by no more than four families living independently of each other, if the owner actually maintains and occupies one of such living quarters as his residence."

I.e. it doesn't apply to my parents renting out their basement apartment. However, many properties on AirBnB do not meet the requirements for this exception because the other does not make the property his residence. Under 42 U.S.C. § 3603(b)(1), a single family house is not excluded from the act (after December 31, 1969), if the rental is achieved through the "use in any manner of the sales or rental facilities or the sales or rental services of any real estate broker, agent, or salesman, or of such facilities or services of any person in the business of selling or renting dwellings . . . ." I don't know if AirBnB falls within that (broad) language (see subsection (c)), but if it does, it would be illegal to discriminate in the renting of a house you own for the purpose of renting it out on AirBnB (which is becoming more common).

In any case, my point is that when you are discriminated against by a hotel, at least you have legal recourse. As you point out, unless an AirBnB host has a property he owns for the purpose of renting out, you don't even have that.