Some of the footwear item descriptions are confusing and misleading. For example, flip-flops’s description says “Simple sandals. Very difficult to run in.” except that they have an encumbrance rating of 0. So from a gameplay perspective, barefeet and sandas are identical except sandals give some protection from attacks.
But should foot encumbrance follow a curve? It would make sense if it did. If the practical effects of encumbrance is a corresponding change in running speed, then barefoot encumbrance should start at a high level (say, around 30) to simulate just how slowly you have to navigate around an environment full of debris when your feet lack protection. Most light footwear should therefore give lots of negative encumbrance, up to a point where the equipment becomes so heavy that it reverses course and finally starts giving positive encumbrance.
It certainly makes sense from a realism perspective. If you find yourself in the cataclysm barefoot, shoes might be one of the first priorities to look out for given how otherwise you’d be required to navigate your environment in a debilitatingly slow and careful manner.