It's not about obligation. Nobody is claiming we're legally obliged to pay developers of free or open-source projects. It's about interest. As in it's in society's interest to pay for valuable infrastructure that's available to the public.
As an example, consider weather data. It's a huge benefit to all sorts of people to have good weather and climate data. It supports not just the activity of citizens, but of many businesses. Are we obligated to pay for that? No. But is it in our interest to? Definitely. If we want things to be sustained, we need to make them sustainable.
Otherwise, eventually the person in Nebraska will decide they have something better to do: https://xkcd.com/2347/