Looked up the article. Granted that Savage doesn't sound like she was as tactful as she should have been*, she is correct in that lemonade really does have an opportunity cost.

Indeed, I think it's trivial to show that Savage's actions were not a result of zero-sum bias. If she were just trying to maximize resources for herself on the basis that there's a limited supply of stuff, she should just accept the lemonade. After all, she'd have both the lemonade and the money then. Yet, she argued for a position that would have made her worse off if she did perceive the situation as a zero-sum game.

Instead, as Savage writes in her reply to the criticism, she figured that the girls would be better off running the lemonade stand as a model business, as it would teach them lessons that would improve their future earning and society as a whole**. Given this belief, her action of trying to convince the girls to accept payment is the "positive-sum thinking" you seem to be talking about.

If you want to say that Savage's actions were wrong for some other reason... you know what, I agree that she just should have accepted the lemonade. But this isn't zero-sum thinking.

*Although contrary to some... creative... interpretations of the article I found Googling for it, Savage seems quite civil, if perhaps eccentric.

**I'm skeptical of the ability of a lemonade stand to have much of an effect one way or the other.