Shouldn't a good worker be rewarded?

I can't figure this out. My son, Jeff, has been working at a kiosk in the food court at the mall since October. He has never missed a day of scheduled work, always gone in when they needed extra help and stayed late when others would not. So why is it than he is being docked hours because business is slow? My son went from working 15 hours a week to 6. He used to work every Friday, Saturday and Sunday. Then Saturday and Sunday and now he just found out that it is his turn to lose a Sunday this week. I just don't get it. Is this how you reward a good employee? There are full time employees that don't show up or call in sick at least once a week. There is no seniority because it just opened last October, so all the employees started roughly at the same time. Jeff is so upset that he has decided to look for another place to work. While filling out his applications he has even written that he would like to give his current employer two weeks notice before starting somewhere else. His father told him that's very generous and that's more than he would do.