No one is comparing or likening charge to some other thing. We simply give opposing charge "directions" a name. Everyone uses the name. It's a convention.
The comparison that is being made is the direction your thumb points when your fingers wrap in a particular direction. It's not merely giving it a name - it's specifically likening the vector to the relative orientation of your fingers. That's why the comparison is memorable. If it were merely a name with nothing that it is compared to, it would not be memorable nor helpful.
http://en.wikipedia.org/wiki/Metaphor
No one is comparing or likening charge to some other thing. We simply give opposing charge "directions" a name. Everyone uses the name. It's a convention.