Yet another post from you (OP) that is full of factual inaccuracies.

The fare rules depend on the carrier and route in question. There are plenty of routes that price as one-way trips (i.e. no minimum stay requirements). There are plenty that have a 3-day/Saturday night minimum (CO loves these). There are plenty that are a straight Saturday night minimum.

Just buying in advance won't necessarily "solve" the problem. There are simply some routes/fares that don't price that way.