Questions about "MEV and the Limits of Scaling" by Robert Miller

My main question is about how can gas fees paid by spam bots not grow proportionally to gas consumed?

I thought Ethereum gas fees are paid directly per unit of gas consumed, so the gas fees paid must grow proportionally to gas consumed?

How do spam bots consume so much gas but pay so little in fees?

I have a question too. where we can find the charts in this research?