They need to add some new name then, when you Backstab with a staff of club.. Backwhack? In any case, staves can be used indeed to "backwhack" someone, and I mean, why not - it's effective when you stab a dagger into someone's back or hit them with a brick/club/staff? Result is the same anyway
And yes, PeeWee is right in that BG2 didn't follow the second edition rules "all the way".