I have a couple of questions here if you'll allow me to play devil's advocate.
1) Given that what Trump is doing is allowed by law - what circumstances can one imagine in which this kind of travel ban would be okay (since it's legal) or is the law itself a problem?
2) I'm curious about (1) because there are legitimate reasons why Trump's ban is more draconian than Obama's, all of which are rooted in various screw-ups by the previous administration. Those are:
a) Last year, the vetting process somehow managed to grant visas to 9500 terror linked individuals, all of whom are missing. When the visas were revoked, there was no way to locate them. (Source: House oversight committee hearings & Mainstream media ~ 2015 / 2016)
b) DHS has 40 staff members currently handling 51000 applications for credible threat to life asylum requests. (Source: House oversight committee hearings & Mainstream media ~2015)
c) Top intelligence officials have named the refugee program as a high risk attack vector (Source: CNN, ~ 2015/2016)
Under those 3 criteria alone, if you are taking over as President and you see this kind of track record from the administration, and you get this advice from the intelligence agencies, would you really be comfortable with the visas issued thus far?
I sort of agree with the principle in theory that if the government has granted someone the right to enter, they ought to be allowed to enter, but that's never really been an airtight thing either. Your final entry is always conditioned upon being interviewed at the airport and getting your passport stamped, and answering various questions - where will you be staying, for how long, etc. etc. I suspect it would have been more principled to have a comprehensive screening process for people when they landed, but that may be an operational nightmare.
Given that there are plenty of PhD students, guest workers, etc. from Iran mostly, who are immensely inconvenienced by this kind of draconian rule, I am not entirely in favor of the policy. At the same time I'm curious as to whether a President is really dutybound to stand by the visas issued by the previous administration when there is proof of this sort of incompetence / error in the process at this magnitude (i.e. nearly ten thousand).