Of course there is more to the argument than that. You talk about the entire industry when in reality, the 5 publishers were around 40% of the market.
-90% of the market (Amazon) set prices. That's okay.
-40% of the market (5 publishers) come together to fight Amazon. That's bad.
-30% of the market (Penguin Random House) decides to merge (aka legally collude). That's okay.
I don't think there is any straightforward right and wrong here.
The law says differently. Like it or hate it. At current - Amazon hasn't done anything illegal. Neither has Penguin/Random House. Apple and the publishers - not so much.
And in markets - 40% can be considered significant. Obviously.