I wonder how long-lasting this "quota" effect is. The study only looked at the immediate effects of moral behavior, not the more important long-term effects.

To make an analogy with physical exercise, maybe flexing your moral muscles exhausts your ability to be moral for the rest of the day, but when you wake up tomorrow your moral strength will be not only restored but actually strengthened. Most forms of exertion I can think of (e.g. learning, writing, working) work like this, so I wouldn't be surprised if the same held for doing good deeds.