Aerobod is correct. They’ll auto-assign middle seats in the back of the plane, because those are less desirable seats, and that’s what a Basic fare is. And it’s the same as pretty much every airline’s Basic Economy seating.

There’s no trickery or punishment. Basic gets you a less desirable seat on the plane, for people who just want to get from A to B, but who don’t care about where they sit.

This still seems to be a source of angst and anger for a lot of WS customers, based on frequent complaints I see when browsing social media. WS customers seem less likely to understand what Basic gets you compared to other airlines’ customers. I think there’s still some potential for improvement in how they communicate Basic features: both seating and cancelability.

I prefer the way Delta handles seating for Basic: it’s assigned at the gate from what’s truly left over. With WS, there’s much angst created when Basic pax are auto-assigned seats in the middle of empty rows at T-24.