Why does the BOP (or is it the DEA?) require pharmacys to store CII's in a locked case or among the other drugs?

Sec. 1301.75 Physical security controls for practitioners. (b) Controlled substances listed in Schedules II, III, IV, and V shall be stored in a securely locked, substantially constructed cabinet. However, pharmacies and institutional practitioners may disperse such substances throughout the stock of noncontrolled substances in such a manner as to obstruct the theft or diversion of the controlled substances.