Here is the problem as I see it:

Ticketing rules for specific fares are objective and easy to understand, as regards minimum and maximum stays, days of week, etc. In fact, it would otherwise be very hard to implement ticketing and fare calculation by computer!

Rules such as United's "Back-to-Back Ticketing" rule quoted above are vague and subjective.

IMHO, the airline can certainly write the rules so as to maximize its revenue, but the passenger, by the same token, ought to be able to use the same rules so as to minimize his or her expenditure. Otherwise the following could occur:

Let's say you were about to spend a month at a client location, with a short trip back home in the middle to pick up the mail, save your marriage, whatever. If you planned this beforehand, you might book something like the following:

1 Oct LGA-ORD
13 Oct ORD-LGA
14 Oct LGA-ORD
29 Oct ORD-LGA

So you have two round trips, both at discount fares. But what would you think if the airline looked at this and said you're trying to evade the fare rules, and the only discount round trip allowed is 1 Oct/29 Oct, with the short trip home in the middle of the month having to be ORD-LGA-ORD, at full fare? Would anybody tolerate this? I don't think so.