As a regular camper there, I was a little bit stunned when I read it.

The exact quote is:

"Pursuant to the provisions of 16 U.S.C. and Title 36 C.F.R. — the following acts or omissions are prohibited on lands and waters within the legislative jurisdiction of the USDA Forest Service, Land Between The Lakes National Recreation Area. ...
Violation of these prohibitions are punishable by a fine of not more than $5,000 for an individual or $10,000 for an organization, or imprisonment for not more than 6 months, or both.
...
7. Possessing or using a mineral or metal detector, magnetometer, side scan sonar, or other metal detecting device, or sub bottom profiler. 36 CFR 261.53 (f)"

Link to the actual page

I've sent them an email asking for clarification.