This is the first time I've ever seen a district judge countermand the entire federal executive branch. I did not know they had the authority to do that. Where was this kind of action over the past 8 years??
There is nothing in the TRO that even addresses the law already in place that authorizes the POTUS to execute such an executive order. I have a feeling that those 3 monopoly businesses have a lot to do with this.
There is not much explanation or reasoning in the TRO, but we can glean some information.
According to the Judge's order, the TRO refers to sections 3c; 5a,b,c,e of Trump's EO.
The reference to Section 5 is pretty clear, being that it seeks to stop any preferencial treatment of minority religions (it's the part trumpeted by some as enabling, for example, the prioritized acceptance of Christians from places like Syria). However the reference to Section 3c is not explained, but from what AG Ferguson has said, it is about freedom of movement and a person's right to due process of law. Many of the affected persons already live and work in the US, whether on green card or temporary visas of some kind, but are currently abroad or would have their ability to travel restricted by the EO. The outcome may turn on whether these persons enjoy the same constitutional benefits as citizens, and the precedents are mostly that they do (http://scholarship.law.georgetown.edu/cgi/viewcontent.cgi?article=1302&context=facpub
The law mentioned, the McCarran-Walter Act, aka USC 1182(f), is what Trump specifically cited in the EO, and as said this has been used by Carter previously. However the role of the judge in this case, and when it inevitably goes to appeal and maybe even SCOTUS, is to decide whether the actions of the EO violates either the Constitution or other statutes which may take precedence. There is no magical process by which laws are automatical de-conflicted or laws are declared 'unconstitutional', but rather this needs to be established through the courts.
So the EO may have violated the Constitution, or it may have violated statute law, or the law under which it was issued may itself violate other statutes or the Constitution, or even if none of the above applies, the EO might violate individuals' rights under the Constitution. The only place to test this is through the courts.