Rangers simply use the Pokemon to their convinience and gain even if the intention is good. That's the thing about Rangers, they just use random Pokemon, then forget about random Pokemon. That looks more like using them as tools than trainers, trainers eventually form a bond with the Pokemon.
I'm not talking about Rangers in particular, I'm talking about the regular people of those regions where the wild Pokemon stay with humans without Pokeballs being used. And even for a Ranger, their primary duty is to protect the environment.
They've never, ever mentioned pokemon being able to leave if they wanted before now (I don't even remember them saying that in B/W, but I'll take your word for it). That's the writers trying to cover their ass, because they realized trying to justify the forced slavery of animals would make the player seem evil.
http://tvtropes.org/pmwiki/pmwiki.php/Main/GameplayAndStorySegregation
Your Shiny Level 84 Charizard you've raised and EV and IV trained for 2 years has decided it doesn't like you anymore. It releases itself. Too bad. What a fun game!
There's a reason why gameplay trumps story or setting in most games.
You know what other RPGs where you have party members who won't leave no matter what action you take? Every single other RPG made in the past 10 years (Bioware excepted). And you know what species those party members are? Human!
OMFG! Final Fantasy is based on human slavery! FREE YUFFIE!
That's the writers trying to cover their ass, because they realized trying to justify the forced slavery of animals would make the player seem evil.
If it's so evil, stop playing Pokemon games. Also, I hope you don't own any pets in real life. You should probably toss them out into the wild. I'm sure they'll like that better. Or you could be like the real N/Team Plasma (PETA) and set out to execute all pets because death is better than "forced slavery"!
Really, the reason N bothers me is that Pokemon ownership is not the biggest issue he should be worried about.
What he should be freaking out about is that people eat Pokemon.
And what's even worse is that other Pokemon eat other Pokemon.
Even though all of the parties are sentient.
That's a MUCH MUCH bigger issue than people owning pets.
N should be going around trying to get people to stop eating Pokemon. And even more importantly - to try to get Pokemon to stop eating each other.