That was until 1956-57 when it changed because the Montreal Canadiens would often score multiple goals on 2 minute PPs.
At the start of this season, the NHL changed the way power plays work. Prior to this season, a team could score as many goals as they wanted in a two minute power play with the penalised player remaining in the penalty box. The NHL changed it so that when a goal is scored on a two minute power play, the power play finished. The reason for this was because the Montreal Canadiens were so dominant on the power play, the NHL needed a way of ensuring parity. The previous season saw the Canadiens score 26% of all the league's power play goals. Oddly enough, the number of power play goals league-wide actually increased from 251 to 265 after the rule changed. Montreal, though, scored 10 fewer power play goals.