I don't think there's any purpose in overselling the socioeconmic thing - I suspect a major reason for these trends is simply that if most of your family and friends breastfeed, then you are probably more likely to do so, and vice versa.

Re advertising, there are 2 issues. One is the impact of formula advertising on breastfeeding rates. Why does this matter? It's a public health issue - we would save tens of millions in public health expenditure every year in more babies were breastfed just to 6 months (I've seen a study somewhere... can't find it now). This isn't controversial. Already 85% of mums in Aus leave hospital breastfeeding, so it's not about forcing those that don't want to. And it leaves room for those that can't. The big gap is, firstly, in support for parents, and secondly in normalising the practice in society.

The 2nd issue is a more general one: is it ethical to push bogus claims in advertising products for infants and young children?

The problem with the 'immune support' claims - aside from being an almost meaningless statement on its own - is that they clearly also associate this with being 'more like breastmilk', suggesting this is somehow comparable to the immunity offered by a mother's antibodies. It isn't. That is an advertising lie - no, maybe it won't hold up in a court of law, but it is still a lie, you know?