Um, I don't remember it this way, at least not from the 1978 movie. How would they legally adopt him when his origin was completely unknown (well, the Kents couldn't exactly tell the county officials about finding him in an object from space that landed in their back yard...)? They didn't get into the legalities in the movie, but presumably they just claimed they had him naturally. Of course, this is lying, but as far as the government is concerned, he should be legal.