Comment by flomble

This is the crux: does it count as stealing to accept the offer of a contract which causes the offering party to lose money?

My answer is no. They offered the service at a certain price, you accepted it. Whether either party profits or not is not part of the contract.

So no theft has taken place. If you want to claim that's it's unethical to take the free money that they're offering, you need to provide a justification for why that is. The onus is on you.

The only way I can see you attempting to justify it is by saying that it involves taking advantage of unforeseen consequences of the contract. But as has been pointed out, they fully intended to lose money, so that doesn't work.