Thanks windscythe! i.e., the 'decaying plant' is returning CO2 (not CO) recently sequestered from the atmosphere, where using fossil fuels adds CO2 sequestered over hundreds of millions of years ... in the blink of an eye, geologically speaking.
Since CO2 makes up such a small part of the atmosphere (about 0.04%), whatever is added is relatively larger than would be the addition of a major constituent like oxygen or nitrogen.
While someone may not like the term 'carbon footprint', or the legal mumbo jumbo associated with it, there can be NO denying the fact that your lifestyle and choices have a huge impact on the quantity of greenhouse gases you add to the atmosphere.
Unless, of course, one chooses to cling to the belief that humanity is incapable of altering the planet's climate. In that case, you can go along with the idea that 'carbon footprints' just help someone justify their job.