You're right, but it's a lot harder to account for emission reduction, and offsets based on those are often illusory. For example, you might award carbon offsets for replacing a coal plant with solar, when that would have happened anyway just because the economics worked out better. If you're actually removing carbon from the atmosphere, that's not an issue.

As a matter of government policy, we definitely need to focus on reducing emissions, and a simple way to do that is with a carbon fee. But for voluntary contributions right now, it's not so clearcut.