Because at the end of the day these offenders are victims of their disabilities. The law is based upon the idea that people who choose to do wrong are punished, but in the case of these disabled people they have no choice. No amount of punishment or treatment will work on them while they have their disability, and it is this same disability that will force their children who inherit this to follow suit, leading to 'bad families' and contributing to 'rough estates'. The benefits are enormous in terms of money saved in addition to having a happier society. Every person given this treatment is not only considerably more likely to be as law-abiding as the average citizen, but any attempts to treat them with anti-crime courses will be much much more successful. The prison system is full to bursting, and each prisoner costs a fortune to catch, convict, and imprison. At the very least this idea is worth a government investigation.
The only drawback is that this would lead to people fretting over "the slippery slope". But this can be remedied with this simple rule: Only replace genes which prevent independence of life with the authority of the General Medical Council. That way only the criminally disabled, the severely physically or mentally disabled (e.g. the blind), and those with dementia etc. would be treated.