I agree that the sentence was light, remarkably so considering societal attitudes. That said, the punishment existed even for cases such as Julians. He was desribed as having social and intellectual disabilities from a young age, which is why his parents sought the resequencing to begin with. Had it been legal for cases where it was necessary (like for children born with these kinds of disabilities), then his parents would not have had to break the law to seek treatment.
I did not see the significance of this the first time I watched DS9 as a teenager, but now, as a parent in my late 30s... if I knew my little girl was suffering and there was a treatment available which was denied by bigoted law... would I break the law to obtain the treatment... Yes, I think I would, as would many parents, and I am a law abiding person as a rule. There are some moral circumstances in which is is necessary to break the law, not because you have no respect for law, but because you recognize that sometimes it is wrong, even when it's right most of the time. Richard and Amsha were not wrong to seek treatment, they did the best they could for their son, they did what they did not because they are criminals, but because they love their son, and they didn't want to see him institutionalized for the rest of his life. As a parent, I get them, I understand their decision, and in their shoes I'd have probably done the same.
I'm not saying at all that the Federation should make it available to everyone, even those who are born normal, only to those with significant disabilities. Such things should be used to reduce suffering, not to make designer babies, and there is a huge chasm of difference. If you could avoid passing on a potentially debilitating gene, wouldn't you? For the health of the next generation, even if it is too late for you? Think about the human side of this.