Why should a balanced scorecard have objectives as well as measures?
Developing objectives before measures prevents premature measure design. This is the tendency for people to leap straight to measures instead of defining more clearly what they want to measure.
If you decide how to measure, before you are clear what you want to manage, you will limit your thinking to what you think you can measure. As a result you will end up managing that which you think you can measure, rather that choosing measures for that which you want to manage.
The problem is that people limit what they think they can measure to very tangible things (money, activity, outputs, and physical deliverables). These quantifiable measures are useful, but they are also a narrow view of the organisation. This thinking encourages greater numbers of financial and process measures and leads to the imbalance that the balanced scorecard set out to address.
Ultimately you do want to measure progress towards your objective. However, this gets interpreted as all objectives should be measures. Defining the characteristics of an objective clearly and then, separately, deciding which characteristics you wish to measure and how to measure those characteristics allows you to be clear about the difference between the objective and its measurement.