To be precise they don't sell carbon offsets, they sell regulatory credits. These schemes force the sum of all cars sold to be lower in emissions than in previous years; the ability to trade credits provides extra incentive to companies which are making an out-sized contribution to the overall efficiency.

The problem isn't that Tesla is selling. The problem is that others are buying. At the end of the day it's not Tesla's fault if these government schemes aren't resulting in ideal overall outcomes.