I can see the point of not drinking alcohol in uniform when off duty but chewing gum?
The point is they were on duty, and serving meals. To be honest, chewing gum while in the galley, out of sight of passengers isn't an issue. However, chewing it in front of passengers (especially the ones that do it like a cud-chewing cow..and I've seen them) while serving meals is just plain bad manners.
You wouldn't (I hope) sit chewing gum in front of you guests at home when they're eating, so why should customers have to put up with it!