Brian,
That's not entirely true. Some card rooms will only allow unlimited raises if that betting street begins head to head. There are situations when two players remain yet further raises are not allowed. Limit $10 and $20 on the turn, 4 players remain, Player A bets 20 Player B raises to $40 Player C folds, Player D raises to $60, Player A raises to $80 (the cap with a 3 raise limit)...Player B folds, leaving 2 players...Back to Player D (action now head to head). Player D raises to $100, but he can not. Player A can not be put in a position where it will cost him more than the $80 cap. He raised knowing that there would be no further raising allowed.