Young children have limited impulse control - they can no something is not appropriate, but not be able to stop themselves doing it. 15 months is still very much a baby (i know you dont like the term, but intellectually, she is still on a huuuuuuuuuuuuuge learning curve, and baby is much more apt than child to describe her)

it's a comfort thing - can you try to replace your boobs with some other comfort item (blanket/toy) - DD needs to put hands down tops - mine, Daddy's, Granma's - it's nothing to do with BF, it's just what she does. we all accept it - and remove it when it's not an appropriate time/place and try to distract her (rather than saying no, we redirect her attention to something else)