First Ladies of the U.S. always have a worthy "cause" during their husbands' Presidencies. Lady Bird Johnson's cause, for instance, was "beautifying America". Laura Bush's cause was literacy (Which is kinda funny, considering who her husband was).
Michelle Obama's cause, so far, is trying to prevent obesity, especially childhood obesity, which has become a widespread problem in this country. Recently, as part of her program, she has articulated the benefits of breastfeeding, because breast-fed babies generally have less tendency towards obesity later in their childhood. The IRS, in conjunction with this, has announced or clarified that breast pumps can be taken as a medical deduction on individual tax returns.
Needless to say, there is nothing that either President Obama or Mrs. Obama can ever say or do that doesn't get a negative political response from somebody, and in this case the loudest response came from Congresswoman/Tea-Party-nutcase Michele Bachmann, who mouthed off that a First Lady advocating breastfeeding is "government interference" in people's lives. Ms. Bachmann, of course, has no issues with other "government interference" -- for instance, the government deciding who has and who does not have the right to marry. Mrs. Obama is not trying to force any mother to breastfeed -- she's just giving advice. There is no Defense of Breastfeeding Act.