My sister tested + to strep B and she went into labour naturally and was given the antibiotics by IV when we went to the hossy.. I think if you are known to have a quick labour resulting in not enough time for drug to be administered then they would prob induce so they had more control over the situation... and every hossy is different in their policy....
I believe strep B is fairly common these days, I read somewhere that the bacteria is in everybody but is present in "some" while pregnant !! not sure how true that is.