BUR Controller described a conflict event between an air carrier departure from Runway 26 and a helicopter operating on Runway 8; the reporter claiming the airport restriction regarding helicopter operations is unsafe.

Narrative

Aircraft X requested a Runway 26 departure with Ground Control & I (Local Control LC) approved this request. In the meantime; I had Helicopter Aircraft Y in my traffic pattern for Runway 8. A couple minutes passed & Aircraft X taxied to Runway 26 and called ready to go. At this time I ran Aircraft X down and momentarily obtained the release from the appropriate SCT sector. Aircraft Y had landed Runway 8 and was maneuvering on the runway approximately abeam the D8/C8 taxiway tie-in. While I did perform my usual scan of the runway before clearing Aircraft X for take off; I missed Aircraft Y and had momentarily forgotten about him. Aircraft X was then cleared for take-off and neither the helicopter or Aircraft X reported anything until Aircraft X was off the departure end of Runway 26. At that time; the pilot informed me that he had just passed over a helicopter and this is when I became aware of the incident. I'm unaware of the exact proximity of the two aircraft; but I would guess it to be within a few hundred feet horizontal. I would like to note that it is extremely hard to see these helicopters while they are holding or maneuvering on the runways here at BUR. For this exact reason; I would also like to note that I have long deemed it unsafe for these helicopters to be required by the airport to operate on the runways as opposed to operating on taxiways as they do at most other airports. Recommendation; once again; I and many other controllers at BUR have long felt that the airport authority forcing helicopter operations onto the runways is unsafe. It's unfortunate that it takes an incident such as this to bring this safety issue to light. These helicopters come over quite often from neighboring airports requesting pattern work and often maneuver over or on the Runway for 5-10 minutes at a time. Most of these helicopters are very small and easily blend into the landscape; which is exactly what happened in this case.