Suppose you think the environmental cost of flying between two cities is 100 euros more than taking a train. Then if someone prefers to fly even with a 100 euro carbon tax on flying, it is inefficient to prohibit him or her from doing so.
Only if the harm can and will be undone quickly with the collected tax.
But also, we prohibit lots acts that are harmful to others, instead of taxing them. It's also a moral thing. Values are changing and increasing share of people are seeing unnecessary emissions as a wrong.
