I have a vaguely similar anecdote. A former employer of mine withheld bonuses from the entire team since I had recently given my notice. They were quite open about the fact that they would give out the bonuses once I had left, and just didn't want me to get a slice of that particular pie.
You'd have to ask them. I'm not a lawyer, and don't have ambitions of being one, but they might have been concerned with such an action appearing punitive.
