Would this mean that you can not take weapon focus:staff to get bonus damage as a wizard or druid for your spells/invocations?
Well, ambiguity aside, I'd say by the intent of weapon focus you'd have to be using a power with the weapon keyword.
Unfortunately, the feat is not that specific.
It is intended that you use the astral fire, burning blizzard, etc. feats to get bonus damage with certain spells, not one feat to get bonus damage to all spells.
It is not clear that using a Staff/Quarterstaff for your implement is one of the ways to use a single weapon focus feat to get bonus damage to all your spells. To me, it's seems that it is not, but others disagree.
I don't believe that there's a strong reason to disallow it
(it makes staves a slightly stronger implement choice, it adds diversity without increasing power).