They're not just counting the emissions that Exxon (for example) produces by extracting oil. They're counting the emissions from when that oil is burned by the end consumer (say by me in a car) and accounting that back to Exxon.
Why is it dumb? Purposely selling a product that pollutes and wanting to shift all the blame to the customer sounds dumb to me. They aren't selling paper weights.
If people stopped wanting to burn hydrocarbons for energy all of these companies would stop digging it up from the ground.
Conversely, if all of these companies were suddenly shut down 100 new ones would rise up in their place to replace them.
Demand drives consumption.
Writing a report about how 71% of emissions come from 100 companies doesn't convey meaningful information to the reader about the nature of the problem of global warming.
People don't want to burn hydrocarbons for energy they just want energy. The proof is in the pudding, if people wanted to burn hydrocarbons for energy there would be no renewable energy sources.
Demand for energy drives a consumption for energy and these companies provide the energy. If we had instead learned how to convert puppies to energy in the late 19th century instead of fossil fuels, we'd probably be burning puppies.
Yes, this is kinda dumb.