SAN controller questioned airport vehicle operations on the active runway's displaced threshold during landing/takeoff operations; claiming a waiver was in place covering the operation.

Narrative

Working combined Ground Control/Clearance Delivery/Controller in Charge Airport Operations called Ground Control and requested to work within the runway displaced threshold. He stated that they have 'waiver' permitting this operation while aircraft are landing. I have never heard of; been briefed on; or read any regulations that permit this practice. Airport vehicles moved onto the runway with multiple aircraft on final. Traffic was called; one pilot questioned the landing clearance; three aircraft landed passing 50-100ft above vehicles in the displaced threshold. Please note: At no time did the AMASS (Airport Movement Area Safety System) tag or alert any of the involved vehicles. No alarm ever sounded which indicated that the AMASS was not operating normally. All aircraft landed without incident. Recommendation; verbal briefings if/when this practice will take place. Explanation of standard operating procedures if/when this will take place. AMASS should be out of service if not tagging or alerting. AMASS should remain out of service until it is fixed and operating normally.