Helper is working on a different standards and we value your time, so we dont want to stop helping your miner to mine more coins if the mine run out of credits. Mine that had run out of credits, will not get credited from your miner as he will not be able to mine anything because nothing is left, but helper is still able to mine credits for some time. We done that in case someone just missed the fact that mine is empty, but evenually you will be stopped and you will need new mine to continue.