Even now knowing about a genetic disease early can help with treatment. Sooner or later gene therapy will be possible. While some diseases may be just as treatable with gene therapy starting after birth (in cases of SCID for which there have already been successful trials) there may be other genetic diseases where treatment must begin in the womb for it to be effective, since much development will have already taken place by the time the baby is born.
The child has a right to health that supersedes the rights of the parents.
A better question to ask yourself would be "What is the premise of mandating genetic testing on newborns without parental consent". Does the government have the right to do something to your body without your consent? No. Only time government has any right to do something to another person's body is when he/she has become a danger to himself/herself or others. Courts though have to approve of the intervention.