US Customs has said it will be used "for vetting purposes, as well as applicant contact information." It has not said what it will be checking for, so you are concluding something which isn't there.

All they ask for is the account name. How do they check private FB posts given a name?

How do you know they will be checking for FB posts supporting ISIS?

What does "supporting" mean? Would "I support the right of people to resist foreign military attacks" be considered support for ISIS?

How long will the information be stored, and who will have access to it?

Is there anything else they will use for vetting? Perhaps they'll go after hackers as well, including these hackers at Hacker News?

Well, I do. There is a whole host of problems and issues associated with it. One is that the border is a murky legal area, US Constitution is not completely applicable there, and the laws that do work are not sufficiently known to the public.