I'm not justifying their actions; I'm trying to answer the question asked and conjecture about possible reasons for the furniture to have been burned instead of donated.
Is it possible the furniture was infested?
Is it possible that the furniture was not in a condition that could meet the standards for an ethical donation? The Goodwill, Salvation Army, and other similar charitable organizations are compelled to spend a substantial amount of money every year discarding donated items that they cannot sell. Furniture is especially costly because of its bulk and the expense of transporting and storing it. Donated clothing and other items that are more economical to ship have controversially been sold to developing countries where it is sold rather than given away to the poor, to the detriment of local retailers. Furniture typically isn't because it's too expensive to ship. But a lot of what is donated ends up being burned or hauled to a dump site, at the expense of the charity. For this reason, there's only a few such organizations in Los Angeles (and likely other cities as well, but this is the only one I personally have experience with) that will pick up furniture donations, and it is on the condition that it is fully functional without any significant stains, odors, or potential problems. They ask a series of questions on a donation form to determine its eligibility, and have the movers inspect each piece before loading it onto the truck.
Even very expensive furniture in excellent condition can be challenging to donate. Last year my parents donated a lot of furniture after they moved, and it was a more arduous task than expected. The move was prompted in part by the Skirball Fire that had caused smoke damage to the house, but none of the furniture being donated had any odors, marks, or other problems that would impede it from being sold. Still, most refused the donations. The only ones willing to pick the furniture up were the Jewish Council and Out of the Closet, and both were so booked it was originally going to take two weeks to set up a date. Through the outreach department at our church they did donate some of the furniture to charities, such as a home for teens and young adults who aged out of the foster care system, but it was at their own expense that the furniture was moved.
People will often place ads on Craigslist for items they are giving away for free, with the condition that the person taking the item do the literal heavy lifting and transportation, but that entails answering a lot of emails and texts, making arrangements, and everything else, and sometimes the people who say they will come don't, or they get there and want to inspect the mouth of the gifted horse.
I can definitely see why it might be more cost and time efficient to carefully burn the furniture instead.
