It's not a straw man - the point is that society defines crimes by "bad acts" (usually in combination with "bad states of mind") that should be discouraged and penalized, even if there's no actual overt harm done to people or property.

The notion of a victimless crime is therefore ill-formed: the purpose of the criminal justice system isn't just to penalize people who create victims - it's created to apply penalties to specific acts/mental states that have been deemed morally culpable.

When people talk of "victimless crimes", they don't actually mean crimes that lack victims - they just mean those sorts of criminal acts that they don't really think are actually morally culpable.