It's not a law - it's a concept, part of the Common Law dating back to 1216.

The principle in contract law is called "Unjust Enrichment" coupled with the
concept of "Acceptance by Silence."

If someone does something that benefits you, you have an affirmative duty to
demurr at the very instant you perceive of the other's action. Failure to do
so implies consent and a binding contract is entered.

If your neighbor cuts your lawn while you watch, you owe him the fair value
of his services. To put it in plain, contract, terms: 1) He offered a
benefit, 2) By your silence, you agree to the terms of his offer.

Ownership of the fence has nothing to do with the derived benefits.

You obviously know nothing about contract law.