Official comment

We have debated this. Ideally, you would never change the values on a drop down box that has been included on a published form. The issue is that a value you delete or change might be essential to the purpose of the form, and you could lose either the opt-in or the sale as people bail out. So we keep multiple versions of the drop down field to maintain that backward compatibility. The initial values, when the drop down field was first published on the form, stay in place.
We could certainly remove that behavior if demand is high. We'll leave this open for votes and comments to see if there's a demand for it.