It's quite simple, really. In most cases, such 2-night minimum requirement is envoked when hotel believes that they have great demand for one of the nights (let's say, Saturday during some weekend event, but there are various reasons depending on the location), but much lesser demand for the previous night. Their yield management tells them that they have a pretty good chance of selling both nights by imposing such a restriction, even if it means turning away some business (like yours). It's always a gamble for the property, and if yield management conclusions prove to be wrong wrong, they will have some unsold rooms for that Saturday night, and they will most likely try to unload those rooms at a discount through some channels at the last moment.