If you receive merchandise that you didn’t order, you have a legal right to keep it as a free gift.
However (for example) Maine's website says:
A person is guilty of theft if the person obtains or exercises control over the property of another that the person knows to have been [...] delivered under a mistake [...] and [...] the person fails to take reasonable measures to return it. Violation of this paragraph is a Class E crime.
These two appear to contradict.
Does this mean this is actually different across states?
Is the FTC potentially giving people advise that might get them in trouble with their state?
What's the rule on whether you can legally keep merchandise that was mailed to you by mistake?