Kids won’t explode if they hear the words ‘no’ or ‘you will have to wait until I am finished looking at the shoes’.
People won't explode if a mother sits on an unused seat in a shop and BFs her child.
Lots of people on her are making out that the issue is where she sat but it's clear that, if she had been just holding her toddler, they would have had no issue whatsoever. Their real issue is that she was BFing, which they think should be done somewhere more 'private', more 'comfortable' or 'more appropriate for eating'. Some think that she should have given him a different sort of food or drink without being able to explain why BM isn't acceptable.
The OP wasn't taking a seat that was needed by anyone else, although that doesn't matter anyway. There isn't one poster on this thread who is genuinely concerned that the OP might have prevented another paying customer from sitting down to try on shoes. It's the BFing that they take issue with, not the sitting.
Those saying she could have made the child wait are missing the point that there wasn't any need whatsoever for the child to wait. He asked for something she was happy to give him there and then. The benefits of learning to wait until other people are ready to give you something don't come into it as she was ready.
The seat she chose felt sufficiently private, comfortable and appropriate to her. She made a decision that it was the right thing to do to agree to her child's request. The facts that others may have felt differently or that some people might be uncomfortable if they became aware that she was BFing are irrelevant.
People who are offended by BFing in public just need to be exposed to more of it until it becomes unworthy of comment. The OP is kindly contributing to that 
Posters scrabbling around for spurious reasons why BFing was inappropriate in that situation, whilst declaring themselves to be pro-BFing, are clearly blind to their own prejudices.
Mothers are free to BF their children where, when and how they choose and the age of the child is nobody else's business. This thread is a clear illustration of how many mothers, even though they may have BF themselves, are far from supportive of BFing in general.
If you wouldn't be concerned about someone giving their child a cuddle or a drink from a bottle or cup somewhere, you have no right to be concerned that they are BFing.