I saw this thread title and was ready to be shocked and appalled by the OPs intolerance, and yet....
IMO - YANBU
Ok, I am pro breastfeeding and I did bf DS until he was 13m. (including on park benches and probably supermarket benches) And I do feel that each mother has the right to find her own way, to do things in the way she feels most comfortable. It is also a pertinent point that we don't know exactly what happened from this mother's point of view.
BUT just because she has a "right" to bf in this way, does not mean that we all have to be comfortable with it, or that we are trolls for having an opinion on it.
The people using the Nuts magazines argument - you are assuming that we all are fine with lads mags! I don't particularly like them, I don't particularly like low cut tops, I don't particularly like builder's bum, I don't particularly like the idea of a woman squatting down with her boobs out over a shopping trolley feeding a child. These things do not have to be mutually exclusive!
We are NOT tribal women! They don't take vitamin supplements in the amazon. They don't have central heating. They don't wear Boden. We're in England. And it's like Libras says, we have a lot that nature has given us, but in our culture we don't necessarily reveal these things in public. It is not unreasonable to feel shocked or surprised at something which we don't see every day. Squatting half naked in the crisps aisle.
It's not about her breastfeeding, it's about not taking her child out of the trolley, and about having both boobs out. It's an image of carelessness. Her choice, yes, but our choice to have an opinion. And by making the choice she made, she chooses to mark herself as a certain type of person.
I would also like to point out that as a "judgy mummy" I don't like mothers who give their children bottles on the move either. It's bad for the child's health to not stop to feed. So don't be too quick to assume that people who are agreeing with the OP are necessarily doing so because they want limits on bfing. It's also about taking a bit of time to do things properly and not being lazy. (And before I hear gasps of shock, I am not saying this woman was lazy, because like many of you say she may have had a desperate need to do something straight away. I'm just saying that feeding is important enough to do properly. And it's contradictory to say that the most important thing a breast does is provide food and to then not consider this job important enough to take a bit of care over.)
Having said this, when DS was a newborn I was so hungry once (and didn't realise until he was firmly latched on) that I had to hold him with one arm while I made a sandwich with the other, it was a very tricky task! So I do get how difficult it can be.