How does this square with the Impoundment Control Act of 1974? That came about when Congress passed funding for certain programs, and Nixon refused to spend the money on those programs. If Congress budgets money for immigration enforcement, and the Executive refuses to enforce immigration, wouldn't that be a case of impoundment?
That just proves that the court is totally political, not to mention corrupt.
Well, while all executive power is vested in the president, the other two branches do have some limited powers that they can exert to coerce the executive. Congress can legislate exactly how the president is to to do things (but generally the president has to agree to this by signing the legislation into law) while the courts have the power to declare actions of the president in violation of the law.
Since the courts have all judicial power to say what the law actually says, the courts can tell the president he president is violating the law. However, this is only if the president's non-enforcement of the law actually breaks the law, then he will be called to task in the courts. If his non-enforcement of the law does not break the law, then the court's don't get a say in how the President does his job.
In the case of the Impoundment Act of 1974, the law said that certain funds were required by law to be spent on specific items. The law itself required the president to spend the money. The president refused to follow the law. Thus the president broke the law that he himself signed. The case of Nixon's impoundment was cut and dry, either the president spent the money on the items in question or he did not. If he did not then he broke the law. There were concrete decisions that were violated and it could be proven in a court of law
However, when Obama refuses to enforce immigration law, the law is much more vague and gives the Executive Branch more leeway in how to go about doing this. Unless, the law says that all illegal aliens must be removed or dealt with in a specific way, you can't actually say that the president is breaking the law. If the president isn't breaking the law, then the courts really have no say in the process.
In short, generally speaking, if you want to sue the president for selective enforcement of the law via the courts, you must be able to prove that the president is actually breaking the law by not enforcing it. (Non-enforcement of the law does not necessarily imply the president is breaking the law.) If you want to sue for selective enforcement of the law, the law must give the executive no discretion in the matter, and it must be demonstrable that the president is in violation of this law by not enforcing it. (Though, I suppose that it could be argued that any selective enforcement of the law violates the 14th amendments promise of equal protection under the law.)
By the way back to your question about impoundment. Yes, there would most likely be a case of impoundment, if the law was specific enough about how the money was to be spent. However, what if Congress assigns money to be spent on immigration enforcement, and the president does spend it, but does not allow it to be spent on the aspects of immigration enforcement he doesn't like but which are not explicitly required in the law?
In order to be effective at preventing this, Congress would have to pass a law so nit-picky and detailed, that it would make your head spin. Everything would have to be itemized and earmarked down to the most specific details, otherwise the president would be able to find a loophole and do what he wants anyway.