The Phoenix's main disadvantage is they don't have the budget of bigger clubs - blame it on the depth of the owners pockets or on the size of the metro or whatever, it's a fact. The main advantage is they have exclusive access to the largest pool of players of any club in the competition, and they have a rule in place locking those players to the Phoenix if they want to stay in the A-League. It's a massive deal if you think about it.
It's prudent to maximise their strengths and develop players for the first team, because it's a captive market kiwi players will be priced cheaper than equivalent Australians (until they're good enough to be foreigners) and they also generate extra money if they're sold which can be spent on the squad.
It really is the most obvious way for the Phoenix to challenge for silverware under current circumstances.
I doubt the club are looking to sell several players every year, no one would want to go through another rebuild to the extent that they are right now, but a player every year or two makes good sense. The Phoenix are a football club first and foremost, selling players is just a way to close the gap to bigger clubs and it happens all over the world.
I don't understand why people are bitter about this other than the fact that they're bitter about life in general.