Because that's what it all comes down to: choice.
If a woman chooses to change her name when she marries, I certainly don't have a problem with that. I do, however, have a problem with people who think women should take their spouses' last name. In modern-day America, women are not their husbands' property. While, fortunately, I'm pretty sure that such a law would never happen, it's frightening to me that so many people think this way--"legally required"! Laws are created to protect people and to protect justice. Legally requiring (i.e., forcing) women to take their spouses' names--besides erasing much of the equal rights work that people have fought for tirelessly for hundreds of years--does nothing for justice.