I'm assuming that there are beaches in Oregon that are state (or city or county) maintained/operated, right ?
This is one of those "funny" questions. Because I'm sure that someone here, if they looked long enough and hard enough, can find things that say (or mean) "no". So too can the same be said, if the same question were asked of CA state beaches. If you looked long enough and hard enough, in the state of CA park's dept codes, you will indeed find "dire sounding" things. And if you went to Sacramento and asked enough state-of-CA archies, you would indeed find a few to tell you that you absolutely can not detect there . Because heaven forbid you might find an old coin.
However, with that said: The reality is, that you can detect state of CA beaches till you're blue in the face, and no one's ever cared. So .... I guess that not enough people have asked the right questions, of the right people. And we're thankful for that !