I wonder if sadistic Zeus would sometimes not give him a boulder to push for a day or two.

I think the catch there is that Zeus doesn't imagine Sisyphus happy.

The double catch is that neither Zeus's nor Sisyphus's state of mind matter, skipping a day or two is detrimental to both the good and the bad interpretation. If Sisyphus is unhappy pushing a rock every day (the first level interpretation) taking it away gives false hope and fosters doubt. If Sisyphus becomes happy or fulfilled pushing the rock, taking it away temporarily removes that satisfaction and again leads to doubt.

In other words, arbitrary and random work, putting someone in an uncertain and unpredictable situation is worse than constant and well-defined work.