Giving up smoking while pregnant can on occasions be incredibly difficult. I smoked 6-8 cigarettes a day before I fell pregnant, and I fell pregnant by accident. In fact, I was enjoying a 'last hurrah' of smoking before planning on quitting ttc, which made it harder.
I smoked when pregnant with DD. I didn't smoke a lot, and I was desperately trying to quit, but it was very, very hard. For the first few weeks when I didn't feel pregnant at all, it felt as if I had nothing more than a small piece of plastic I had peed on telling me to quit smoking. Then when I started throwing up 6-10 times a day and the hormones really kicked in, strangely the 1 cigarette a day I had cut down to became almost impossible to give up. Selfish, yes, but I felt like absolute hell, I wasn't eating, I was an emotional wreck (we had huge issues with having to move out of our flat and buy a house during my pgy), I didn't go out and socialise because of the sickness, and that one cigarette in the evening felt like the only enjoyable thing in my life. I smoked more in the week we moved out of our flat because of the stress. I actually had a few cigarettes, the first in a long time, when I was at home in early labour.
I felt guilty the entire time but somehow that never quite stopped the odd occasion I would give in. As someone mentioned above, some women completely go off the taste of cigarettes - and some women don't. Some women almost crave them more. And that means that sometimes they give in and have a cigarette. That was me. I still smoke now, though in the evenings, out of the house, in an 'smoking coat'.
Everyone is entitled to their opinions and plenty of people will judge those of us who do smoke when pg, or when they have newborns. But nicotine is an addictive substance and for some of us, despite all the help available and all the guilt, cigarettes can prove nigh on impossible to completely give up.
So, OP - no, there is no justification. But there are explanations from some of us. Each woman, and each pregnancy, is different, and each woman will struggle with different issues.