Re: allergic to breast milk?!
Here is my even if they aren't allergic to your breastmilk per say, they can certainly be allergic to something you are eating and in my honest opinion that would then in a round about way make them allergic to the milk they are drinking (your breastmilk). Yes it may not be to you milk exactly but it is still going to cause all sorts of problems unless you eliminate the offending food.
I think some people may use the allergy excuse because they don't want to breastfeed and get sick and tired of people trying to put them down for it. Then there are some babies who have allergies to a lot of stuff (or intolerances) and maybe the mom can't easily eliminate all of the offenders and maintain a well balanced diet in which case breastfeeding may not be the most beneficial for either party.
FWIW I am a huge advocate of breastfeeding and have breastfed all 3 of my children HOWEVER, I have dealt with allergies and intolerances that have made breastfeeding mighty complicated. I never judge another mom for not breastfeeding unless I know the entire story (and not just the story from a third party).
Feel free to disagree stongly
Though we have the courage to raise our daughters more like our sons, we've rarely had the courage to raise our sons like our daughters. -Gloria Steinem