If you enabled a seat heater, enabling hardware that the car already has installed, or hot-rodding the engine, I don't think it is that big a deal.
But if you downloaded and installed software from tesla that didn't come with the car, or did something like enabling free supercharging, that would be more like theft of services.
I expect if this becomes a thing, features will have to be downloaded after purchase.
1) If tesla took possession of the used car and then sold it to you, I think they can disable features.
2) If you sold your car to someone, and THEN tesla disabled features I am not ok with that.
With case #1, I think it is like any used car. People flip cars. They can take a car, remove expensive rims or other options and sell the car without them. People also buy cars and part them out, selling each piece individually. This is ok because the flipper owns the car before selling it and they can do what they want.
If the seller didn't tell the buyer, or the seller themselves didn't know - who's fault is that?