> A slaver to his slave: Your attempts at achieving freedom are undermining a business model that is letting a lot of people enjoy cheap clothing, and that would be selfishly making the world worse.
This ignores harm of slavery to the slave, but in both the online ads case and your burger hypothetical there are no analogs to the slave.
> I genuinely cannot understand how refusing to consume something makes me selfish.
The argument is pretty straightforward. In your hypothetical, hunger has been mostly solved: everyone can eat as many burgers as they want. But this solution is fragile, and rests on people consuming the lettuce that comes with the burgers. By choosing to remove your lettuce, you increase the risk that this falls apart and we'll be back to the status quo where some people are hungry, and others spend a substantial portion of their income on food.
(I still think this is a silly hypothetical)