Adding this in means you are taking away from people via added "code theft" from them.

Everyone powers down at some point at least a few times here and there. Adding a punishment to it so other people profit from them powering up or down is nonsensical and the opposite of the spirit of what this blockchain is.

The same aim could be achieved by making it so there is 5% reduction to total rewards given out from the pool and it wouldnt cause any direct theft.