Curious on your thoughts about Kroger/Great Value/Archer Farms (Target) brand items? At least Amazon marks them clearly as house-brand, Archer Farms is practically camouflaged at Target among the brand names, and it's placed pretty prominently too.
Personally I think it's hard to draw a distinction between the two cases - either they're both fine or they're both not. My line would be closer to: if Amazon noticed a certain thing was selling really well, created their own version of that thing, and then de-listed the original thing from their store, that seems like the point at which it becomes anti-competitive to me.
Store brands are an age-old practice so it's interesting to revisit them in the context of mega-corps like Amazon. Although WalMart+Great Value is probably a similar scale.