First of all, may I commend you on your accurately addressing the issue at hand: carbon credit OFFSET.
Why DO they want to pay for carbon sequestration by, in this case, farmers? Primarily it is because those companies employ some not-so-green practices and could pay for someone else to sequester carbon in lieu of improving their poor environmental practices.
In fact, if they are so interested in “reducing emissions,” why would they need to be purchasing carbon offset credits? A cynic might suggest that these interested parties have found, or are certainly exploring the possibility, that they can more economically buy carbon offset credits than to address their own environmentally unsound practices.