Chinese maritime claims have no basis in law, but they want to maximize their legitimacy anyhow. One avenue is through military dominance, which is very powerful but invites resistance/blowback. By adding claims of intended economic exploitaion, they get a rationale which is non-threatening, and can be used by anyone who wants to avoid conflict with China to say "see, their aims aren't so hostile after all."

They don't have to make the charge go away, they just need to FUD it.