BE40 First Officer questions the appropriateness of the Captains descent on the PAPI while executing an instrument approach procedure.

Narrative

Briefed the approach noting the step down fix. Captain concurred and reviewed the approach. SoCAL approached vectored us onto the final and cleared us for the approach. The Captain crossed BEVEY and descended. He initially saw the airport and began to align with the runway. I called course deviation and buildings in front. He returned to the course. I called level at 1150 and he stated he was visual. I called level we are cleared for approach. He stated he had the PAPI. I called level 1.5 miles to go and he continued to descend stating he was visual that he was above the PAPI. We were not cleared for the visual approach. He descended onto the PAPI and proceeded inbound to a landing at SMO. The AIM states that a visual or contact clearance is necessary for a deviation from approach limitations. The distance remaining from CULVE requires more than 1000/FPM descent to the runway for a landing. If the overlay included distance to the runway end it would assist the pilot in planning. NOS charts are used by the company.