"They" is Congress. Congress makes the laws - not executive agencies like the National Park Service.Why would they care about people looking for and finding gold nuggets and artifacts? If they're all that worried about them getting away, why don't they dig them up themselves or have some archeological group do it. No point in having those items buried under a lake. It doesn't seem logical to me at all.
If you don't like the law (I don't) you will need to write your congress critters and ask that the law be changed. Or you might considering asking them:
Why would they care about people looking for and finding gold nuggets and artifacts?