Yes, it may be rude but some people will take third party criticism about manners better than if it comes from long-suffering family members. If you don't think that is true, just think of when a money topic comes up at home-family might not agree or even acknowledge issue but a stranger having same conversation with a person might get an acknowledgement for having a different opinion.
Cellphone conversations in stores-it seemed worse in video rental stores as quiet contemplation went out the store. Now I hear people talking on their earclips in grocery store aisles and for some reason, being in the line for cash does not inspire speakers to end conversations for some quiet in readiness to talk to cashier/acknowledge him/her.