buying alcohol and cigs???

So i just dont understand why when i go to the store with my mother, that they wont let her buy it without looking at my i.d., okay thats stupid since im her daughter, and they wouldn't i.d. my little 14 or 17 year old brothers so why me, considering im her child, its not like were buying the stuff for me, or minors.... sorry they just irritate me each and everytime i go there and i almost want to stop shopping there cause they are just so ignorant...