BF and smoking cigarettes??

I was reading about smoking cigarettes while breast feeding and from what I read doctors actually would rather a smoking mother breast feed then formula feed as they’re still more benefits to breast feeding even when smoking then with formula fed babies. I personally don’t smoke, I just have been really into breast feeding this time around and have been reading so much about everything breastfeeding. What are your personal inputs with this? I genuinely wonder if what I’m reading is true. So many people I know have BF when also being a smoker and I was always so curious about this. How wouldn’t the nicotine affect the baby enough for doctors to discourage it?