Okay, here are my replies on the initial topics raised in this thread:

1) Back to back ticketing: There is nothing raising any "antitrust" issue by the mere contractual limitation by ONE airline of passengers and agents issuing or purchasing tickets on its airline. All it is is an attempt to "price discriminate" among passengers, depending upon each person's perceived demand for the ticket. For example, the Sat. night stay has evolved as the airline's easiest way of distinguishing between pure business travelers (like lawyers who have to be in a certain place at a certain time, no ifs ands or buts about it), and those whose demand is more price-sensitive (i.e., leisure travelers, persons with enough freedom to reschedule business appointments in another town, etc.). Similarly, other fare rules are being used to price discriminate in the same way, like advance purchase tickets, non-refundability, etc.
Back to back ticketing is simply a method of attempting to avoid the sat. night stay. If the conditions of the fare are part of the contract between you and the airline, the airline claims a right to enforce the limitations. however, as a practical matter, it is awfully difficult for the airlines to police against a particular individual (although they threaten to take away your FF miles!!). Horrors!!
Because the travel agents have a more continuous relationship with the airlines, the airlines try to enforce these rules by charging the "correct" fares against the agents, to scare them out of ticketing.
As I see it, however, the airlines are going to have harder and harder time enforcing it, with everyone's access to on-line ticketing themselves, and amultiple sites through which to do so (Expedia, ez-sabre, etc.)
There is an issue regarding price discrimination by the airlines that might raise issues under the Robinson-Patman act, which purports to outlaw price discrimination, but for that discussion, you might want to sit in on an antitrust class.
As this one is getting kind of long, I will post it, and deal with the coordinated fare and travel agent commission questions in another post.