Is it not allowed to offer cheaper gas prices in a bundle?

Inspired by the recent gas price threads, I’m wondering why gas stations don’t offer lower prices for gas when it is bundled with other purchases- It seems the gas stations make no profit from gas alone, so why not offer discounts to customers who spend x amount on their higher margin items? I’m sure I’ve seen some sort of discount like this somewhere in the past, but it seems like this sort of thing should be everywhere now that people are desperate for any way to get cheaper gas. Why isn’t it?

I know my local Kroger’s supermarket often offers a discount on gas to people who purchase a certain amount of merchandise in the store. It’s not a huge discount, maybe $0.10 per gallon or so, and it requires a $100.00 in purchases over the course of a month using their loyalty card, so it’s not a huge deal, but it is a discount.

In L.A. there are some gas stations that have a car wash. They offer gas cheaper to anyone purchasing a car wash.

When we owned a Marathon station, we made more money on a candy bar than we did on a gallon of fuel. Most of the stores here have “frequent buyer” cards that reduce the price per gallon based on purchases made.