Why does a Bella cargo bike not act like a car driving in reverse?
You are not wrong to ask how tricky these things are to steer. They are very disconcerting to manoeuvre until you are quite familiar with them.

They certainly would be unstable at 30+ km/h. But good luck with that; you'd need to be quite an athlete to achieve such speeds. If you could pedal fast enough though, I think you would have a quite hilariously dangerous vehicle in your hands.

In practice, they are ridden at much lower speeds, where the rider doesn't find themselves caught out in ever-greater cycles of steering over-correction, and where the outward movement of the back end isn't rapid enough to be destabilising.