You can get your votes back by clicking on something you've already voted on (unvote?) , so you can then revote on something else.
The idea i think is so they find the most asked for feature.
Broken system.
Inevitably ideas at the time of announcement when there was a huge surge of registered members are the ones that get the most votes. Once you've casted all your votes would you actually go back there and search for more ideas? If you see one that you happen to like you'd be racking your brain which ideas you have to unvote just to be able to cast again.