Because....?
Because the state wins either way.If you treat a kid who's not your biological child with any degree of decency you're deemed to have taken on parental responsibility and the courts,in order to preserve that father-like relationship,declares that you,and not the Department of welfare,must support that child.
At the same time if you treat a kid who *is* your biological child like dirt then the courts declare that because he/she *is* your biological child you,and not the Department of Welfare,get to support the child.
So..heads,they win...tails,you lose.