I think it comes down to the accountability of the party actually entering into the contractual obligation. In this case, it would have been the Canada Border Services Agency. They ultimately are accountable for the decisions that they made.
I think the more you funnel everything through one department, the more you will slow down procurement. PSPC's role is to encourage competition and to follow the many rules that exist in procurement. The ultimate decision rests with the department that makes the final call. In this case, it would have been CBSA.