Yahoo Employees Forbidden From Playing Fantasy Sports

Interesting: Yahoo employees have apparently been banned from playing fantasy sports on any site. “As for Yahoo’s new rule, employees company wide, it seems, will no longer be allowed to play fantasy sports on any other fantasy site out there. Previously, the limitation only applied to fantasy sports leagues on Yahoo.” I’ve gotta wonder: is this legal? I mean, I’m sure you could create an employment contract that stated it, but can you retroactively forbid your employees from taking part in legal activities? I honestly don’t know. Not HR certified, not a lawyer…