Wow, reading some presumption into this article, aren't you? Why do you say there was "no way?" The one really obvious way they could believe it was "protected" if not owned was if there was a developer's agreement describing the lake as "common area," which is a very typical way to develop in Florida. Lake is made when fill is used to level lots, homes are built around lake, developer maintains lake for a period of time then conveys to Property association. The average property owner does not monitor the ownership of the common areas and as long as the developer still has record title, tax notices will be sent to him and not the homeowners. In this case, there was little to protect the homeowners apparently.
A different question is raised on what to do about it. Posters have suggested eminent domain, adverse possession and easement theories, the correctness of which depends largely on one's politics.
Still, it seems unfair to "blame" it on the homeowners without more facts suggesting their culpability.