Maybe I'm wrong, but I suspect this is only an issue in RWC year as the timing of the internationals, and the small amount of time that the PI players get to train together (another issue, I know), normally means that there's no/little conflict between the ITM cup, and test rugby for PI players.
Things may have changed a bit with the compensation for All Blacks, because I remember when Ben Franks left the Tasman Makos a few years ago, the Makos didn't seem to upset because of the lack of games that Franks actually got to play for them, and the savings that could be made by contracting a non All Black/ability to use those savings elsewhere.
It's interesting that these comments come out of the Auckland Rugby Union, who seem to be one of the richer unions in NZ ... some unions, such as the Makos, operated below the imposed salary cap for years (not sure if they still do), and have a distinct lack of international players, and seem to select their players on the basis of who's going to provide the best value for them, not on the player's availability for a particular nation
Well, there's an element of risk involved for any player in the NZ rugby environment. All players that want to represent NZ, must play rugby in NZ, and. as you rightly point out, not everyone can play for the All Blacks, but the players must make the choice, and forgo the opportunity to earn better wages elsewhere. Many other test playing nations have a similar selection criteria involving a requirement to play in their domestic competition (to varying degrees), if a player wants to represent their country, but there's no requirements on the PI players to do so - they can play anywhere in the world, taking advantage of the best deal in monetary terms, and still play for their country, so it's not all disadvantages for PI players.
Building "a warehouse full of rugby players" may seem unfair from one point of view, but is it fair to leave/lose other players that could play for NZ, because there's no room in the warehouse, because the space is taken up by players that can't play for NZ?