You know how the FMDAC got that list, don't you? They merely sent letters out, to each of the 50 states park's dept's headquarters, with a question like: "what rules, if any, are there, for metal detecting at your state's parks?" It was the same method for a book called "Treasure Laws of the United States", written decades ago by a fellow named "Grimm". In his book, each of the 50 state's replies are re-printed, on their own letterhead.
While that sounds like a noble thing to do (I mean, who better to ask
than the states themselves, right? DOH!

). But the problem is: put yourself in the desk-bound bureaucrat's shoes, who fields an inquiry like that, in some stuffy state capitol office: Admittedly, within your state, there will be some parks that are certainly historically sensitive. But then there are also probably many many other parks in your state that are not historic monuments. Ie.: simple beaches, turf, or other innocuous places where
no one cares. So if
you are tasked to answer a letter like this, what is the easy answer?

You certainly could not go into volumes of detail, like: "yes at this park, but no at that one, and yes on this side of the lawn, but no - not at the historic cabin at the corner of that one, etc... etc...." So notice that most of them, in their 1-sentence answers, say "inquire at each kiosk" or "with permit", or simply "no" etc... It was the same with Grimm's book when it came out decades ago. It had many people, in the various state's scratching their heads saying "Since when? that's funny, I've/we've detected the state parks here and never had a problem. Now all of the sudden I'm supposed to ask at the kiosk?" And when they'd ask at the kiosk, you can imagine the downward spiral, where .... rangers who probably never even gave the matter thought before, or would have ever even noticed, now get asked "mother may I?" And now they can just morph anything like "don't disturb the vegetation" or other such nonsense, and tell you "no".
I can't comment on all the other states, but just looking at CA on that list ("need permit"), I can tell you that there are a lot of state parks here that get routinely hunted (as long as you're not a nuisance or snooping around obvious historic sites), and no one is bothered.