They make the plan; they report on the plan. If they can't deliver on what they came up with, would you not say that they will be held relatively accountable for that? It's not as though the plan is imposed on them by the PCO or the PMO or somebody else; they make the plan themselves; then they are the ones who report back on it and hence need to be accountable for it.
Would you not say that it puts them in an awkward position if they can't deliver on their plan?