Rules are for safety and fairness. This rule is no different. An athlete is allowed to have a phone during the race. However, it must be put away, out of sight, making no sounds, and not being used. Like headphones, they do not have be “in use” to get a penalty. they are not allowed to be in your possession. If you need to use it in case of an emergency, pull off the course and use it. Safety----A cyclist or runner listening to music, taking pictures, or playing on their phone is a distracted athlete and a danger to themselves and those around him/her. Fairness—an athlete can use the phone for pacing during the race. This would constitute an unfair advantage.
USAT Rules: 3.4i–Unauthorized Accessories (personal audio device) 3.4k–Unauthorized Equipment 5.8–Illegal Equipment (cycling) 6.3–Illegal Equipment (Run)
All are a variable time penalty.
I know that some running races allow them. But my feeling is that is more about appeasing paying customers.