Must have been a SureFire, with the Streamlight as just a prop
Not necessarily Surefires- see, on TV even the darkest gloomiest scenes are usually quite brightly lit on the set- lots of blue lights usually, with the odd red or orange to stimulate a streetlight or 2. Then either in post production, or during actual filming, the "brightness" of the picture is turned down. And to make the bright white torch beams stand out, very bright spotlights are used off-camera.
Sure, the actual torch usually does give its own light, especially in closeup scenes, but in some cases where they need more light, they can get re-inforced. I saw some movie or other where they were using Maglites, and it was plain to see that there were faceted MR16 spot lamp bulbs (probably 50-75 watts!) in the ends. Presumably a hidden cable came out the back to a beltpack battery, or just trailed on the floor. Needless to say, the beams were wide and flawless!
Probably no trouble for a Surefire as it is, maybe that`s why that one CSI episode used M4s- they wanted brighter light?
Ah, the magic of TV.