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The dark pattern here is that you don't actually have to choose a seat. You can click they disabled-button-colored continue button, click through the warning about not choosing a seat, and avoid paying for the seat choice cost (on each leg there and back).

As a solo traveller, you may not care. As a family you really don't want to be seated away from your kids. If you don't pick seats, will you get seated together? Who knows. Depends on if enough other people who want to pay to pick seats leave enough seats together for you... on a popular flight, that's doubtful.

So it's not just people choosing to pay for a luxury, it's a tax on people who don't know they can skip the step, and on families who may know, but can't roll the dice.