Think politics is a brutal, hateful business? Try motherhood. And look at the flood of triumph and frustration that poured onto the Internet after Rhode Island hospitals announced they would stop giving out formula gift bags in maternity wards. The “ban the bag’’ movement is a cautionary tale for anyone hoping to sell public advocacy through browbeating or guilt. It’s an impressive feat when the people promoting infant and women’s health - with the best of intentions, undoubtedly - manage to turn themselves into the bad guys.