Having made a stupid decision to begin with, just because she didn't compound her mistake by getting an abortion does not magically transform unwed teen aged pregnancy into a positive event. There is no positive message in it. It is a situation to be avoided, the result of bad judgement and carelessness that will have a negative impact on the life of the child. It is not to be glamorized, regadless of who the mother or grandmother is.
If I recall correctly, the statement was that Bristol was going to “convey a positive message about teen pregnancy”, not that she was going to portray teen pregnancy as a positive choice. There is a difference.