To be honest, I think the answer to that question is sort of obvious. People think Pokémon is slavery because, on the face of it, that’s kind of what it looks like! We keep wild animals in tiny balls and make them fight each other, for goodness’ sake. That’s really the whole reason I used to talk about this stuff quite a lot; because it’s worth talking about. It’s worth thinking about why Pokémon training ‘works’ in the franchise’s internal logic; it’s worth thinking about what makes it okay and why. It’s also worth questioning how far Pokémon really benefit from the current social order, which I think is something Game Freak want us to be asking: look at the conclusion to Black and White 2, where N is more or less convinced of the rightness of Pokémon training as such, but is now committed to “[freeing] Pokémon and humans [my emphasis]… from the oppression of Pokéballs” – I think what that’s meant to suggest is that there was, is and always will be something intrinsically right about that partnership, but that at some point in human history, something went wrong, and Pokéballs have something to do with that. To me, this is the whole reason Pokémon is interesting! Superficially, it has these obviously problematic themes that need to be dealt with, and you can deal with those very effectively if you dig deeper – but deeper still, and you begin to suspect that maybe something is wrong here after all, something much more subtle that crept up on them and caught them unawares, long ago.
Suggested further reading: this webcomic. It’s mostly comedic for the first ten pages or so but it gets very interesting after that; trust me on this one.