I always shoot with only the ambient light (no flash). This is critical for achieving shots that truly represent what is on stage. If you
PR people don't have a background in shooting for theatre or they are trained as journalists that may be why they like the flash. It also could be if they are shooting film.
Most film is color balanced for daylight so shooting in the theatre under "tungsten" light causes a yellow cast on the image. They make tungsten color balanced film and you can also use color correction filters to avoid this but as film photography seems to be a dying art people are less familiar with this and the products are becoming more expensive and hard to come by.
On the other hand these photographers my just feel like shooting indoors without a flash is outside their comfort zone. They may have just been trained to use a flash indoors.