I don't agree with the explanation of まだ~ない. That fits only for non-volitional verbs. As for volitional verbs, まだ~ない expresses that the action can be done(the condition is already fulfilled), but it's not done just because of the agent's will. See the difference between まだ食べない and まだ食べられない or まだ窓を開けない and まだ窓が開かない, for instance.