I would like to know which planet you are living on, in which it is constitutional for Obama to make the order, and unconstitutional for Trump to rescind it.

Congress makes laws. A president may clarify laws if necessary. Obama simply exempted nearly a million people from a law passed by congress under the guide of 'clarifying' the law. That is law-making by the president. It would be like exempting a million people from the law against robbery or drug trafficking because applying the law to them would be mean, in the president's personal opinion. A president CANNOT do such a thing. 

I'd also like to know on which planet the pardon of Arpaio is unconstitutional, unless you count the editorial opinion pages of the NYT as "the constitution" because that's the only place I've seen anyone argue it is unconstitutional, and they even admit their argument is based on a never-tested legal idea. Never-tested = total speculation upon which no court has ruled.