Why were the 9/11 hijackers allowed into the country in the first place?

When it comes to 9/11 no one ever seems to ask one important question: Why were the 9/11 hijackers allowed into the country in the first place? Everyone seems to assume that the whole world is allowed to come here and nothing can, or should, be done about that.

I remember 9/11 very clearly. I remember debates about how much freedom we should be willing to give up for security. I remember the Patriot Act containing lots of provisions for spying on Americans. I don't remember any talk of a plan for curtailing Muslim immigration or repatriating those that were here already. Later I remember a brouhaha about the FBI and NYPD spying on Mosques. I don't recall anyone asking why there were mosques here in the first place.

We are so brainwashed in this country that we can be counted on to always ask the wrong questions and debate the wrong points. Pro-war versus Anti-war. 9/11 truth versus debunkers. But even the staunchest anti-muslims could be counted on to cheer-lead for more war, and ignore thousands upon thousands of Muslims imported into their own backyards.

Why?