There is another explanation that can resolve the question on Tosafos. In fact, when one derives monetary benefit from another person’s possessions he does owe him money for the benefit. But when there is no loss to the owner, we do not consider as if the benefit resulted directly from the owner’s possessions. Since he did not steal the item, he only has to pay the owner for a benefit that we can say belonged to the owner. When there is no loss to the owner, the benefit is not considered to be his. But when there is a loss to the owner, we say that the benefit is his and that if the squatter derived a monetary benefit he must pay the owner for it. This is because one only must pay for a benefit that has a monetary value, not for a benefit with no monetary value.
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