Because the Executive Branch is a co-equal part of the Federal government and many laws that the Congress passes are broad in scope so that the Chief Executive can administer according changing conditions. So, as long as the Executive administers the law in the context of the law it is lawful.
Look, I know we don't agree as we look at Trump and Obama, but Obama's DACA was a clear violation of this directive. He is on video many times speaking before Hispanic audiences telling them that there is nothing he could do concerning non citizens who came here as children and somehow make them legal.
He is on record saying this time and time again, telling them that Congress needs to change the law and then he can do something. Then right out of the blue he came out with this Executive Order creating DACA, which is in truth a change in the immigration law and that is not a power that he as the President has, not just him but any President.
Up to now and with his Executive Orders, President Trump has not done anything like that. If you have any examples that President Trump has done such things, I am ready to listen.