To begin with, I don't think the game should give you access to a resource at all when you've planted a city on it.
In real life you need to make choices; do we leave this field a cotton field or do we build houses for our population? Can't do both.
In the case of a Great Person improvement the game does prevent access to the luxury, eventhough it's only one building. But planting a whole city can be done while sparing the resource?
And of course no single worker needs to spend a single drop of sweat to harvest the cotton once there are houses on it, because the fundaments of buildings are already automatically doing this, in accordance to the intuition of civ players...