The Constitution says you can't pass laws or levy taxes without congressional approval. Barack Obama does not see the Constitution as a limit to respect. He sees it as a target to be destroyed, and his directive to the administrative state is to find a way.
Find a way to govern as we want, without having to bother with Congress. Do it via regulatory and administrative rulemaking if that's what it takes. But do it.
Apparently the Treasury Department has found a way to do it in the form of a grab for more estate tax revenue. They can't technically raise the rates or establish a new exemption threshold, but no problem. Instead, they'll simply redefine how they calculate the value of your assets for the purpose of taxing them. One way or another, they will get your money.