Are you saying that the people who answer "no" to the question "Are you a US citizen?" need to be removed?
There are millions of people lawfully in the US who are not US citizens.
Not to mention the tens of thousands of foreign tourists who might be present in the US during the census.There are an estimated 13.2 million green card holders of whom 8.9 million are eligible for citizenship of the United States. Approximately 65,000 of them serve in the U.S. Armed Forces.
Not being a US citizen doesn't mean you're therefore an illegal alien.
And no illegal alien will ever answer thruthfully any question regarding their legal status.
They could just say they're a US citizen and nobody would know if it's true or not.
Same way the US Customs never caught a terrorist who answered thruthfully the question about being a terrorist (or a spy) when crossing the border.