You could call it a "preference", but I would then ask, what would be the alternative(s) you envision? Lixes and lemmings could always walk through even 1-pixel-tall tunnels, and more importantly they do have the ability to walk up zigzagging builder staircases instead of turning around near where two builder bridges meet. If physics is altered so that terrain pixels/cells directly overhead can affect walkability, how do you envision reconciling that with the two currently-walkable arrangements I just mentioned? Does thickness of ceiling gets taken into account for example? Are they maybe only blocked from walking forward if the step goes upwards in presence of thick ceiling directly overhead, but not if the next step forward is flat or goes downwards?
In other words, walkability for lixes and lemmings is already a little different from real-life, so you're bound to find some cases where the observed behavior might feel a little off to one's brain, given the mismatch with real-life kind of physics.