AT ABOUT 0933 MST, DEHAVILLAND DHC-6, N76GC, (OWNED & OPERATED BY CANYON AIRLINES) & BELL 206B, N6TC, (OPERATED BY HELITECK) COLLIDED WHILE ON SIGHTSEEING FLTS OVER THE GRAND CANYON NATIONAL PARK. AFTER COLLIDING IN FLT, BOTH ACFT CRASHED & BURNED IN THE TONTO PLATEAU/CRYSTAL RAPIDS AREA. DRG THE INV, NO REASON WAS FND TO EXPLAIN WHY NEITHER FLT CREW WAS ABLE TO SEE & AVOID THE OTHER ACFT. THE ACDNT OCCURRED IN AN AREA WHERE THE DHC-6 MAY HAVE BEEN IN A TURN. THE BELL 206B WAS NOT EQUIPPED WITH AN INTERCOM OR PUBLIC ADDRESS SYS TO COMMUNICATE WITH PASSENGERS; THUS, THE PLT MAY HAVEPERIODICALLY TURNED HIS HEAD DRG FLT TO NARRATE THE TOUR. THERE WAS EVIDENCE THE FAA FAILED TO EXERCISE ITS OVERSIGHT RESPONSIBILITY OVR FLT OPNS IN THE GRAND CANYON AIRSPACE & THAT THE NATIONAL PARK SVC INFLUENCED THE SELECTION OF ROUTESBY THE SCENIC OPERATORS. SUBSEQUENTLY, THE MODIFICATION & CONFIGURATION OF THE ROTARY-WING ROUTES RESULTED IN THEIR INTERSECTING WITH THE ROUTES OF GRAND CANYON AIRLINES NEAR CRYSTAL RAPIDS.