If one of these creatures can’t attack its player during a given turn (perhaps because that player has left the game, or because a member of the opposing team has gained control of it), it may attack another player, attack a planeswalker an opponent controls, or not attack at all. If there’s a cost associated with having that creature attack its player, you aren’t forced to pay that cost, so it may attack another player, attack a planeswalker an opponent controls, or not attack at all.