I do not agree that it is unethical. To call it unethical is removing all responsibility of the distributor/consumer. Before anyone engages in a business opportunity, it is their responsibility to evaluate if the cost of goods sold, franchising fees (or sales quotas), and overhead will be worth the investment to make money.
It parallels buying a loto ticket. The possibilities of earning are discussed, but averages ignored. The supplier makes a lot of money while the consumer mostly loses money (or maybe makes a few bucks).
I think the blame for "unethical" MLMs should be focused on uneducated consumers hoping the business can make them rich.