Says who? This StackExchange is for people writing fiction. They can do whatever the hell they want.
It's fine to advise against certain things that might detract from credibility, but telling people they "cannot" do things comes across awfully self-important.
EDIT: Maybe I'm getting downvoted because of the self-importance of telling people they cannot tell people that they cannot do things? :)
Says who? This StackExchange is for people writing fiction. They can do whatever the hell they want.
It's fine to advise against certain things that might detract from credibility, but telling people they "cannot" do things comes across awfully self-important.
EDIT: Maybe I'm getting downvoted because of the self-importance of telling people they cannot tell people that they cannot do things? :)