If you were to pull up most of the ordinances that "banned" firearms from parks, many of them pre-dated State Preemption.
The question of wasted money was probably addressed back when the State passed "preemption" and all these local jurisdictions most likely decided to just leave everything in place rather than go to the expense of a special Code Revision. They'd deal with it as the issues came up and only recently has this occurred.
Even thought the "laws or rules" are there it doesn't make them enforceable. Just like all those laws that might remain on the books that make selling a mattress on Sunday illegal, or requires someone to lead a motor vehicle with a flag by day or lantern by night, an unenforceable law is just that, unenforceable. To arrest and prosecute would just mean a major expense to the city/county. But then, so would replacing all those signs, just to remove one phrase.
When I see the "no firearms" on a park sign I just smile and "Carry On". Haven't been challenged yet.
A last note on parks, I WOULD like to see some vigorous enforcement of the Leash Law and Dog Crap laws though. Those regulations weren't preempted by State Law and also are present on almost all Park Rule signs.