Target would be better off staying out of hot button political issues.
Actually this is a pretty smart business move for a couple reasons.
For one, GLBT people are usually wealthy or at least middle class. Prime shoppers for target. This is the same reason why some stores also have signs in spanish - tons of the population are hispanic, and it helps to get their money.
Secondly, this is smart for marketing down the line. It is obvious that GLBT rights including marriage will be the norm in most (if not all) states within the next 10-20 years. What is going to play out better for marketing in the future? "Look at us! We supported civil rights when it wasn't popular to do so!" which gets major marketing points. Think about companies that publicly supported/went along with racial segregation. Can you name many? Probably not because they aren't in business anymore. "We used to support racial segregation and had a whites only line!" doesn't play well with the public these days.... GLBT rights will be the same way. Being on the wrong side of history is bad for business. For example, ever hear of a company called N.K. Fairbank? They aren't around anymore. Used to be a soap company that had an advertisement with a white kid holding a soap bar next to a black kid saying "Why doesn't your mom wash you with fairy soap?". People thought it was pretty funny at the time, apparently not anymore.
Third, it helps target attract/retain better talent - especially at the corporate levels. GLBT people are generally smart and creative. Good examples of this are companies like Pixar and Disney that even have gay employee groups at work because there are so many GLBT people working there. Almost every major company in america has rules against GLBT discrimination, offers domestic partner benefits, etc. This helps target remain competitive as far as employees go.