I think this was explained before. But basically the processing takes so long (over a year) that you are guaranteed to overstay most visas and leaving the US is considered as abandoning your petition for a green card.
So you are in this limbo state where you are given some kind of status while the application is pending and you absolutely can’t leave the country.
But that’s not the issue here nor why people are being detained.
People came to the US with intend to marry and apply for a green card on non immigrant visa. For instance British citizens on a visa waiver can just show up and get married. That’s legal.
But it turns into immigration fraud if you always intended to apply for a green card afterwards.
What you are supposed to do is apply for a K1 fiancé visa which takes another year.
So you are in this limbo state where you are given some kind of status while the application is pending and you absolutely can’t leave the country.
But that’s not the issue here nor why people are being detained.
People came to the US with intend to marry and apply for a green card on non immigrant visa. For instance British citizens on a visa waiver can just show up and get married. That’s legal.
But it turns into immigration fraud if you always intended to apply for a green card afterwards.
What you are supposed to do is apply for a K1 fiancé visa which takes another year.