Ok.
First of all, I had problems with low-supply. Huge problems. We got there in the end (it took 6 months), with a lot of hard work, loads of great support from DH, LC and friends. And I did have to supplement until then. We were lucky enough to get donated milk, but I am aware that it is very unusual were I live, and that not all mums would be comfortable about it.
Second, I live in a place were breastfeeding is the norm - at least on paper (you are expected to breastfeed).
However, I do "judge" mums who formula-feed. I am much too aware that most of them didn't try very hard, thought it was a hassle, and started giving top-up bottles within a week or so. My personal pet peeve is the "I didn't have enough milk, he cried at night from hunger, so we had to give him a bottle", when the baby is a porker who was gaining very well already before the first bottle.

: Or the "Oh, I couldn't cope with feeding every 3 hours, I fed her every 4 hours. You do know that your baby empties your breast in 10 min, so there's no use in feeding longer."

:
It doesn't help that society is pro-breast-feeding, it just makes the mums who "fail" (and there are many, as there is a lack of support for any detailed breastfeeding advice, except "Feed on demand") much more defensive. Mums "fail," get depressed about it, get defensive, and aggressively pass on their breastfeeding/formula advice to other mothers, who as a result are more likely to "fail".
Although, there is "classist" element in it, I doubt it has much to do with work (pumping at work is very supported here, although very few mums I know of have tried it). The mums who are most likely to give formula and go back to work are middle class women, particularly in high positions, because child care costs are quite high you generally need a high income for it to matter, and in that case you often need it to pay the mortgage. "Poor" (don't have a better word for it, don't want to offend anybody - I mean, we are definitely poor, at least money-wise) people are just less likely to have a good diet themselves, less likely to get good medical care (an economical choice - as in "I choose to smoke 2 packs a day and go out drinking twice a week, but we can't afford to take our child to the doctor"), good ante-natal care (lack of info maybe, or lack of care, as ante-natal care is free), and less likely to educate themselves.