I bet if you researched the origination of such a ban, you would probably find that it's a response to a decade or two of well-meaning md'rs, who go seeking clarifications, inquire as to the rules, etc.... At a certain point, when enough people become a bullseye, it's only a matter of time before some archie begins to "answer the pressing question".
There's an actual example of this in Utah, years ago, where at a certain point, an edict (a policy or whatever) came down to the rank and file rangers of all the state parks there. Like a "memo" detailing the department's stance on the use of metal detectors in Utah State parks. It went on to list dire sounding things. What was most fascinating, was the opening sentences to the letter/memo, telling the reasons for the memo: It started with the introduction something to the effect:
"The Utah state parks dept receives many inquiries each year regarding the use of metal detectors in state parks. The following will serve to outline the department's stance on this practice...."
And it went on to detail severe restrictions. But when you read such an introduction, you have to see the implication, that apparently BEFORE these "inquiries", apparently detecting had just gone on, and no one had cared (barring unless you were a nuisance in some other way). But given enough "inquiries", well gee, we "better pass this by our legal dept, our indian counsel, our staff archaeologist, blah blah blah". And sure enough, you can guess the answer. Sheesk, it's as if we can be our own worst enemy, eh? I bet this TVA thing is probably has similar psychological self-fulfilling roots.