The point is that when most people use the word, they mean it in the sense by which most other people understand it. That's sort of the purpose of language, and people who engage in conversation implicitly accept the responsibility to seek to understand what their fellow conversationalists are saying, so long as the latter are not committing errors in their use of language.
For example, when the average person says "conspiracy" they mean "ill-intentioned plans made in secret," not "people doing things together". The latter definition renders the term useless, because it is not distinct from the many other terms descriptive of human interaction.