There are a variety of issues with backyard breeding of rabbits.
1. There is a serious overpopulation of rabbits. Breeding prevents rabbits in shelters from being adopted and can contribute to their euthanization.
2. Irresponsible backyard breeders often have no idea of the health risks associated with breeding rabbits -- there are many threads here where ignorant people decided to breed rabbits and ended up with sick parents and dead and dying babies.
3. Genetic issues - a truly responsible breeder is tracking their stock and seeking to eliminate health concerns from their rabbits. Randomly throwing two rabbits together does not accomplish this.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry