As per the previous comment, most of it comes from the way the laws were written. If immigration law says “members of a terrorist group can’t get citizenship”, then USCIS has to have a process for following it. In addition to background checks, they ask the question.
In terms of booting them out, the question makes it much easier. If they are a member of a terrorist group and they said “no”, well it’s an open and shut case of lying on an immigration application. If they didn’t ask the question, sure they could still boot them out, but it may be a prolonged process.
But doesn't proving that the person has lied also require additional efforts, and if that person will contest the desicion in court, can become a long process?
I would argue that proving somebody lied on an application would be easier than proving something like “this person is a danger to the US”. The 2nd is much subjective.
To prove that he lied on the application form, they would have to legally prove him as a terrorist. The legal process of proving someone is a terrorist would be same. So isn't it pointless?
In terms of booting them out, the question makes it much easier. If they are a member of a terrorist group and they said “no”, well it’s an open and shut case of lying on an immigration application. If they didn’t ask the question, sure they could still boot them out, but it may be a prolonged process.