HN is less simple than you think: you can't vote or make polls until you have a certain level of karma; you may be hellbanned; you can't downvote responses to your comments; comment karma is displayed as the number (max -4, real-karma), but still calculated below that; there's a delay -- not a constant delay, but exponential based on the nesting length -- of time after a comment is posted before any replies can be made. HN is complicated; it just doesn't make it obvious to the user. Going along with other design choices that have been made, pg might implement this by simply dropping votes that are made without loading the page; the user would never even know this happened. It wouldn't make the complexity on the user any greater.
And yes, spambots or human spambots can click the link first: it is trivial. But -- assuming this is a problem; I don't know that -- if some don't know about it, their votes wouldn't count. Problems don't have to be solved in one step; lessening them is useful.