This is gonna be a long one, so buckle up.

The original "71%" report by CDP can be found here. The emissions are counted cumulatively since 1988, and all of the 100 companies are fossil fuel producers.

In the case of gasoline, we know the emissions from the EPA that combustion releases 19.59 lbs CO2/gal and the AFDC recognizes 23.7 lbs CO2/gal as the full lifecycle (including drilling, refining, and shipping). So for gasoline, 82.7% of the emissions are the direct result of burning. For other fuels which require less processing like diesel and coal, that ratio is higher.

So you could argue that those companies only made 12.3% of emissions since 1988, and their customers are responsible for 58.7%.

Should you, though? That is much harder to answer, because asking who is responsible for what means now we've exited the realm of science and entered an ethics discussion.

Let's say a person (let's call him Jeaj Valjeaj) steals a loaf of bread. Crime, obviously. Straight to jail. But let's say he was stealing the loaf of bread to feed a starving family. Now we say he's a good guy. But let's also say Jeaj had the money to buy the bread and stole it anyway. Believe it or not, straight to jail. But let's also say Jeaj had the money but the baker refused to sell to him. Well, now it's getting a little convoluted, but it kind of feels like the baker had it coming.

The throughline in this scenario, the one thing that keeps flipping your opinion, is who has the power. If Mr. Valjeaj is stealing bread for fun, or if he has the money, then he could just not steal. He has the power to change. If the baker is refusing business to a starving family, then the baker is the only one with the power to change. And of course in the original story, society itself holds responsibility for placing Prisoner 24602060451 in a position where he has to choose between the law and his starving family.

So let's bring this conclusion to the question of fossil fuels. Is the consumer responsible for burning fuel, or is the oil company responsible for selling it? It depends on which consumer we're talking about. Some consumers have the power to consume less and simply choose not to. Others do not have that luxury -- they must consume to survive, and so the responsibility lies on the influences that shaped our society in such a way as to force people to choose between the environment and a starving family.

This is the direction most philosophical discussion of environmentalism goes. But in terms of praxis, it is a dead end. We can say the individual is responsible for emissions if they have a reasonable ability to change, but who defines "reasonable?" I can define that limit for myself, but I can't effectively judge someone else. More importantly, how can the consumer truly know the environmental impact of their decisions if the producers are constantly lying through their teeth about it?

I'd also like to take this opportunity to attack the concept of ethical consumerism in general. Even if producers couldn't lie or greenwash their products, whenever a corporation does something unethical, it's typically because the ethical thing would cost more. That's the law of externalities at play. If a producer eliminated environmental damage in their supply chain, their prices would rise, their margins would shrink, and they would not match the breakneck rate of growth of their less ethical competition. Likewise, a consumer who only consumes product made ethically spends more money on fewer products, limiting their influence on the market. It's an uphill battle, unless the suppliers are universally forbidden from making that unethical product, in which case it's a level playing field again.

In short, even in ideal circumstances with full transparency, ethical consumerism can only affect change if there is truly overwhelming demand for ethical product, and if the vast majority of consumers have the power to not consume if the ethical product isn't there.

TL;DR: Yeah, those 100 companies wouldn't make emissions if nobody purchased, and it's important to emphasize personal responsibility, but it's difficult (if not impossible) to hold those consumers responsible in any way that's both effective and fair. Supply side regulation is simpler and more effective, particularly when corporations have as much power as they do.