While it may happen on the same stage, it doesn't happen with the same people. I've done back stage theater work, and live sound work, and no audio guy would be running lighting above something static, so we're still talking about multiple people. Whether it's DJ and lighting, or DJ and photography, or DJ and chocolate fountains, it's more than just the DJ. The direction at that point is immaterial to us if it fits into their business plan.
Not so, once you add fresh fruit and chocolate you are a food service provider and that is a regulated business activity. Why not put the DJ in charge of the carving station as well?
I once did a wedding where the catering was done by a DJ turned restaurateur.. so inexperienced was the staff that they took over 2 hours just to get a raw bar setup... this while the shell fish waited in the 90 degree heat. I love the raw bar.. but I passed on this one.
Photobooths are not in the same vein, as it doesn't need any particular talent other than patience.
That kind of expectation is exactly why problems happen.
I don't disagree here. But the same could be said for many other occupations, like my Dentist who offers Botox services.
Do you suppose that is helping or hurting his profile as an authority on dentistry? How much respect do you really have for his knowledge and expertise with Botox? Chances are he is now perceived among his peers and parts of the public as second rate at both. That's the risk when you start adding things that don't logically relate at the vocational level.