The value of what airlines, along with hotels and car rental companies, sell can vary greatly according to the day of the week you chose to sit in one of their seats, or sleep in one of their beds or drive one of their cars. They want to realize the maximum price they can get and try to do that by various means, one of them being to price more dearly what business travelers are after than what leisure travelers are after, since the former is generally more able and willing to pay more for what they are after. Requiring a Saturday night stay over, something most business travelers are unwilling to accept while leisure travelers are not so unwilling to accept, or even want, is one way to segment the market and get more $$$ out of their customers.

Now if someone wants to fly A-B-A on a less expensive ticket but doesn't want to spend a Saturday night in B, can any CoC require that they be physically present in B on a Saturday, preventing the customer from flying, busing, driving, walking, bicycling, or traveling on still other means to C, D or wherever and absenting themselves from B? If so, I'd like to see the contract language. All that the carrier can do is penalize you if they catch you doing what the OP contemplates, that is flying on the same carrier A-B-A and B-A-B to get around their Saturday night stay requirement.