I think that politeness is a good default, but it's a terrible way to change what you see as an injustice. It was impolite for a black woman to sit down an a bus in the south 50 years ago, and being impolite helped change a social injustice.
It is a more common, and completely different, objection to calls for "discreet" breastfeeding is to argue that discreetness is extremely subjective. Some people here have said that is discreet to use a breastfeeding top but no covering blanket, many have said that the baby's face and the whole breast must be covered in order for breastfeeding to be discreet, others have said that the breastfeeding must be in a secluded corner or room in order to be discreet, and yet others have said that the only form of discreet breastfeeding is to do it in the privacy of your own home. Even if you think that avoiding offending everyone around you is an extremely important part of promoting social order, there clearly isn't a single standard for socially acceptable "discreet" breastfeeding.