> I once tried to book a flight with the same-day return; the price for the return flight was mad expensive. The same return flight was a few hundred euros cheaper if you booked the initial flight a day earlier.

Minimum stay durations, like advance purchase restrictions, are a common part of fare construction.

> That's also why none of this is advertised: it's not a discount, but a "we think you can pay more, so we'll charge you more" type of thing.

A surcharge for X vs a discount for ~X is the same thing, it's just how it's presented.

Technically all of this is advertised, it's published in GDS. People just don't really want to read the fare rules because it's boring and a ton of reading.