When I used to fly for business (depart and return within the work-week) I observed that ticket prices were much higher than if a weekend was included at one end or in the middle. I was told that it is because business travelers will pay more. I think that is outrageous! Why should the airline care if I depart on Tuesday and return Thursday of the same week, or the next week? Only because it is business travel?
Why should the airline care if I depart on Tuesday and return Thursday of the same week, or the next week?
and you answered:
because business travelers will pay more.
That's all there is to it. It's called price discrimination and it's practiced by just about any business that can get away with it -- all the more so in competitive industries with small margins, like airlines.
As a first approximation, the airline sells each ticket at the highest price they can get for it, not caring that it's a different price on a different day, or even on a different seat on the same flight. If you're willing to pay a maximum of 100€ to fly, but I'll pay 110€ for a seat with more legroom, why should the airline not charge you the maximum you're willing to pay, while giving me what I want and getting an extra 10€.
It's just the same: if people are willing to pay an extra 10€ to fly from Tuesday to Thursday, then they're going to charge an extra 10€.
Your difficulty is thinking that flying-on-Tuesday and flying-on-Sunday are the same product. They aren't, and we can see that because some people want one but not the other.
There is not such "rule" in the airline industry. It really depends of the airline's policy and the destination flights. Other than that, weekdays are usually busier as more people fly and there's always a huge demand for tickets. Less travelers decide to buy last minute ticket during the weekends, for example. The airfare goes down during the weekends to certain major metropolitan areas and business destinations and the hotels charge less as well. It is exactly the opposite if one wants to fly to any resorts.
So it is not because business travelers "will pay" or "are willing to pay" more. It is because there is a higher demand for tickets during the workweek. People just have to fly to certain destinations and most of them prefer to get back home by Friday.