[mikeash]

I don't think that contradicts the other comment. They said that the hijackers all entered legally. You're saying that some didn't have legal status on the day of the attack. Both are true. That some of them overstayed or didn't meet the terms of their visas after arrival is interesting, but not very actionable unless you want to set up an internal passport system or something.

[masonic]

  I don't think that contradicts the other comment.

Sure it does; "were all here on legal visas" is false.

  That some of them overstayed or didn't meet the terms of their visas

Even those that did have legal paperwork all obtained it via fraudulent means by lying on their applications as to purpose of visit; technically speaking, none was here legally.

[mikeash]

The point was that ignoring people with visas would not be compatible with a goal of keeping terrorists out. I (and I think the other commenter) don't think it's a good idea or worthwhile, but it does fit the stated goal.