I BF and have never ever in my life so much as asked someone FF why they do it, not even asked in a polite way, I mean, let alone interrogated them or challenged them or accused them of being lazy or anything like that. I'd describe myself as quite political about BF though - I have very strong opinions about the politics of it, resources, how mums are so often let down by poor support and so on, but all really on a statistical scale if that makes sense. At an individual level, meeting an individual mum who is FF (or BF) I'd always assume they could have all sorts of individual reasons for their decisions, just like the ones quoted, medication, HIV, family issues etc. I think you have to know someone really well to be sure that you know all their circumstances and starting knowledge, and to say confidently that in the same situation you'd have made a different decision.
I have to believe in these BF mums who are apparently out there slagging off FF mums and accusing them to their faces of being lazy, or interrogating them as to their reasons, in the same way that I believe there are people out there who'd e.g. shout out racist remarks when I wouldn't do that myself. But despite knowing dozens and dozens of BF mums, having been to years of get togethers with mixed groups of mums feeding different ways etc., I've yet to actually witness anyone being interrogated like that, ever. I have seen defensiveness and volunteered justifications just when the topic comes up so I do think that must happen sometimes too.
I think what makes these things so difficult in part is coping with the possibility that people we know are secretly thinking we've made a bad decision. It's hard with any parenting decision to be really thick skinned and not care what people are thinking even if we know 100% our decision is right for us. For instance I once spent 20 minutes waffling away to a friend I know to be very anti-BF about why I was still BF my two-year old - she didn't actually interrogate me at all, or say anything negative, but I was so sure she was thinking bad things about me she might as well have done, as I still came out with all the defensiveness and explanations. Knowing her views, on BF in general, I really wanted her to see where I was coming from and not go out and immediately roll her eyes to her dh about what I was doing. Ironically, despite the fact that I was entirely defensively motivated I probably came across as someone being arbitrary evangelical about BF. I just didn't want her to think badly of me.
Anyway, if I met someone and I knew (somehow by magic, as there's no way you can really know this) that we shared 100% a particular opinion of the health value of breastfeeding or the risks of formula, i.e. we basically started off identical, then I would probably also be puzzled if she didn't even try to BF. But I'd probably conclude there was some reason I didn't know about that she couldn't talk about. When I meet strangers who say they didn't/won't try to BF I tend to assume that another very likely reason they don't could be because they don't share my opinion as to the relative risks and benefits of different methods of feeding - so therefore circumstances that wouldn't be enough to push me into formula feeding (because I'm resisting harder) do lead them to make that choice. I tend to think that different starting opinions of the relative values of the different feeding methods account for an awful lot of different decisions (whereas the support and information issue accounts for most of the forced choices).
After all, the message pushed most strongly to us by the NHS etc., is "breast is best but it's your choice and you may choose to formula feed blah blah" - not plain "breast is best" at all - and if you paraphrase that message it's basically saying there's not that much to choose between BF and FF. In that context, reasons that may seem trivial to someone who's convinced breastfeeding is much better or formula is dangerous actually make perfect sense. Most people's decisions make sense in their particular context.