Not every free app has in-app purchases. My city, Seattle, has an app called Find-it, Fix-it where I can report potholes or abandoned cars. One Bus Away is an app that shows bus arrival times. I have grocery apps that show me what's on sale and give me digital coupons. None of these apps have in-app purchases.

If no apps had in-app purchases, there's no guarantee that Apple could still pay those salaries.

Mind Sharing the reason for downvote? That is what Apple state in their reply in multiple outlet and emails to developers.