They have argued that it, like abortion, is an issue of choice. The babies do not have a say in the matter, therefore the parent's should not be allowed to decide for them.Babies also don't get a choice about wither to be chopped up and vacuumed out before they are born either but apparently that is fine as long as they go into the medical waste bin with their foreskins intact.
As a Libertarian(more and more so by the day) I believe that the government doesn't have a place in peoples business as long as they aren't hurting anybody. I'll even admit that the whole issue of infant circumcision is something that might fall into the whole "hurting anybody thing"(I'm unconvinced that I was hurt as an infant but hey maybe I don't know what I'm missing), but if circumcising a baby could fall into this area killing the same child who just hasn't gotten out of incubation definitely falls into this.
Much as the tyrannical busybodies of the left want to "let us eat cake" they seem to want to have it too.