The NHL was looking at revenue. The amount of money NBC paid for the US TV rights seemed to imply that they would be giving the NHL a ton of exposure, and in the first year or two of the deal it seemed like they were headed in that direction. Having games 3 and 4 on NBCSN probably didn't bother the NHL at that point.
Now however I agree with you. The NHL needs to man up especially during the playoffs and advertize the hell out of the games, and for the love of pete put more games on NBC, especially in prime time on the weekends instead of this 12 noon **** they love since they don't have to pre-empt their affiliates afternoon infomercials and saturday night filler.
Part of the reason games 3 and 4 are on NBCSN is because NBC still wants to put pressure on cable providers to carry NBCSN, and to get viewers to subscribe to cable. It's literally fan punishment for the sake of the almighty dollar.