The short answer is that the block is only temporary. Although the federal district court in Hawaii has granted a TRO (which is a procedural device, not to be confused with a ruling on the constitutional merits), it has not squarely ruled that Trump's travel ban is unconstitutional. This week's ruling only means that the federal court found enough evidence to push the pause button while Hawaii's case against the Trump Administration makes its way through the court system.
Once this case makes its way through the court system, the million dollar question is whether the ban itself will ultimately be found constitutional. Contrary to popular belief, the Trump Administration might actually win the argument on this one, and here's why:
Both the first and second EO's draw their power from the Immigration and Nationality Act ("INA") of 1952 (later amended by the INA of 1965 and the Immigration Act of 1990). The INA says a lot of things, but of particular interest to our current debate is Section 212(f) of the INA, which provides:
(f) Suspension of entry or imposition of restrictions by President
Whenever the President finds that the entry of any aliens or of any class of aliens into the United States would be detrimental to the interests of the United States, he may by proclamation, and for such period as he shall deem necessary, suspend the entry of all aliens or any class of aliens as immigrants or nonimmigrants, or impose on the entry of aliens any restrictions he may deem to be appropriate.
This is the section of the INA that Trump is basing his EO on. In order for his EO to be found unconstitutional, the courts are either going to have to rule that (A) this statute, which was written in 1952, is no longer good law; or (B) this statute is still good law but Trump has exceeded his powers under it. There are pros and cons to arguing either option, but what should be noted here is that neither the federal court in the state of Washington, nor the 9th Circuit, nor this federal court in the state of Hawaii have squarely taken this section of the INA head on.
Now, to be fair, the main reason why the courts have not tried to rule on the constitutionality of this section of the INA is because they weren't being asked to. They were only being asked to rule on a TRO. That said, however, part of the TRO analysis is to predict the likelihood of success on the merits. Therefore, the courts should have addressed this section of the INA, even if only to address it as an academic exercise.
It's unlikely that any federal court, including the Supreme Court, will rule that Section 212(f) of the INA is unconstitutional. Per Time:
In 1981, President Ronald Reagan used it to bar “any undocumented aliens arriving at the borders of the United States from the high seas," while in 1986, he used it to bar Cuban nationals, with some exceptions. In 1994, Bill Clinton used it to bar anyone in the Haitian military or government affiliated with the 1991 coup d’état that overthrew the democratically-elected president. Ten years later, George W. Bush used it to bar corrupt members of the government of Zimbabwe from entering the U.S. And in 2012, Barack Obama used it to bar hackers aiding Iran and Syria.Trump, however, appears to be the first president to apply a blanket ban to everyone from a specific country (more than one, in this case) since President Jimmy Carter used the provision to keep out Iranians during the Iran hostage crisis.