Air carrier inbound to HLN was refused a Visual Approach by ATC after reporting the airport in sight; later conversations revealed conflicting IFR traffic in this non-RADAR environment as the reasoning for ATC's insistence on the ILS procedure.

Narrative

Approximately 20 NM south of Helena; Montana on V21 Salt Lake Center handed us to Helena Approach. After the Controller queried our position; due to the non-RADAR environment; he immediately issued the DME ARC for the ILS Runway 27. As we were in visual conditions with the aerodrome in sight; we inquired about the Visual Approach; but he insisted on the instrument procedure. Unfortunately; with the late assignment and confusion; we joined the arc from well inside. After we were established; the remainder of the flight was uneventful. Cause; after shutdown I spoke with the Helena Tower Controller. He explained that he had traffic directly below us on V21 and so the only way he could descend us; in his non-RADAR environment; was to assign us the ARC for the ILS. He understood that Salt Lake Center handed us to him too late for a smooth transition. Suggestions; I think a company publication; like a quarterly bulletin; discussing the unique nature of an Approach Control; in a non-RADAR environment; concerning conflicting traffic could condition other pilots to be prepared for an instrument procedure despite being in VMC with the airport in sight.