I can't really comment on the justification behind the policy, I can just explain how it works.
Point is, with a few exceptions (notably, H-1B), coming to the United States on a non-immigrant visa with the intent of staying is an inadmissibility (7A1 immigrant without an immigrant visa). It doesn't matter if you're married of have children. Coming to the United States on a non-immigrant visa and then adjusting status due to a change in circumstances is allowed.
>Is it United States policy that the children of such marriages must be raised only by a single parent?
If it's a bone fide marriage (and if there's a child, it probably is), the US citizen spouse can petition for an immigrant visa (which will turn into a "green card" on admission at the border). Immigrant intent is not an issue when entering on an immigrant visa.
My personal opinion that this immigrant/non-immigrant visa distinction should be removed from US immigration law, but right now it's there, so these are the rules right now.
Point is, with a few exceptions (notably, H-1B), coming to the United States on a non-immigrant visa with the intent of staying is an inadmissibility (7A1 immigrant without an immigrant visa). It doesn't matter if you're married of have children. Coming to the United States on a non-immigrant visa and then adjusting status due to a change in circumstances is allowed.
>Is it United States policy that the children of such marriages must be raised only by a single parent?
If it's a bone fide marriage (and if there's a child, it probably is), the US citizen spouse can petition for an immigrant visa (which will turn into a "green card" on admission at the border). Immigrant intent is not an issue when entering on an immigrant visa.
My personal opinion that this immigrant/non-immigrant visa distinction should be removed from US immigration law, but right now it's there, so these are the rules right now.