[tomp]

The main rationalization for this kind of policy seems to be "we already pay for heavier luggage, so why not for heavier people as well?"

In my opinion, overweight costs for luggage have nothing to do with "fair price", but all with "profit maximization" - airlines have to offer some luggage allowance (either by law or to make people actually fly), but they know that many people are careless and will cross the limit. Therefore, they can freely charge them without stirring up too negative feelings about greed. But that's exactly what it is - it has nothing to do with actual costs. Ryanair and EasyJet (though especially RyanAir) earn huge amounts of money this way.

Allowing them to charge heavier people would just inspire more greedy behaviour.