The International Air Transport Association (IATA) regulates the rules of international air traffic. This is done through an international agreement that dates back to the 1920's and is known as the "Warsaw Agreement." Commonly, IATA rules are referred to as "the Warsaw Agreement," as in " The Warsaw agreement says this or that."
Since the beginning, bikes have been allowed to "fly free" as checked luggage as long as they conform to baggage standards.
Unfortunately, many, most, maybe all ticket agents don't know the rules or just don't want to deal with a bike. (For some reason, golf clubs, surf boards, skis and other sports equipment don't seem to bother them.)
Domesticly, airlines can make any bike rules they want, but they must follow IATA's rules on international flights.
I carry a copy of the IATA regulation in my back pocket when I travel with my bike and "whip" it out when the ticket agent gets "snooty." Most of these agents are poorly trained and can be "cowed" easily. They have heard of IATA but are unfamilar with the regulations and thus give in quickly.
I can e mail the regulation to anyone interested.