Always puzzled me too. I found it on Sodnagel's List of TV Crime Show Cliches as rule number 5a.
However, they do get points for attractiveness.
What would happen to evidence collected at a crime scene by a female with long hair that is tossing about? There are many flaws in the program, but the flashlights are a rather small one. As to Grissom's (the character as written) positioning in the spewing of liberal agenda, he claims to be a scientists, nothing more, yet he is constantly tossing out putdowns of conservative 'faith' notions using sound byte swill.
I asked my criminology professor, a Houston police captain with many years in homicide, about the flashlights and he just laughed and said it was a TV show.