This.

If you have studied economics, this practice is called price discrimination. It's ubiquitous. Businesses try to charge buyers more when they are price-insensitive without losing sales to price-sensitive customers. This is normally done by setting up hurdles which the price-sensitive will jump but which the price-insensitive will not jump. Advance purchase is one hurdle. Other airlines used to require a Saturday night stayover for the lowest fare.

If you think about it, this practice benefits the price-sensitive customer. Without price discrimination the low prices would have to be higher, assuming a competitive market. For example even though I occasionally have to pay full fare I benefit tremendously from this practice on average. Customers like our member Ink, not so much.