The point of the complaint is that they were able to do this due to illegal collusion.
And even if people buy a lot of junk food, they might have bought competitors’ junk food. Laws are still laws even if you don’t like the people the laws protect.
Exactly pricing discrimination (i.e. selling at different prices to different customers) is absolutely legal and is market efficient in a market with multiple sellers and multiple buyers.
Pricing discrimination combined with monopsony(single large buyer) or monopoly ( single large seller) powers is not market efficient. It leads to higher prices by end consumers. Price discrimination via collusion + Walmarts monopsony in grocery industry violates that 1930s act and is illegal
Is that the point? The illegal collusion was with walmart to keep their prices artificially low compared to everyone else. They weren't colluding with coke to raise all soda prices.
And even if people buy a lot of junk food, they might have bought competitors’ junk food. Laws are still laws even if you don’t like the people the laws protect.