I understand the OP is making a more general observation, but I don't believe this instance is a good example of it.
Obama wasn't trying to get the crowd "riled up" -- they booed when he said Romney's name, and he told them to turn those emotions into tangible actions.
Was "revenge" the best word? Maybe not, but unless you think the situation was staged for him to get that word out there, I think this is another example of spin 101 . . .