It's the Law
Homeland Security Restrictions
Recreational boaters have a role in keeping our waterways safe and secure.
Observe and avoid all security zones, including restricted areas near dams, power plants, etc. Do not stop or anchor beneath bridges or in the channel. Violators can expect a swift and severe response.
That said, I regularly fish, and see other fish under bridges and have never been questioned, or asked to leave by a LEO.
There's a chance I'm wrong but the way that's written it seems more applicable to waters used as transportation, not lakes. I can't think of a scenario in which a boat tied to a bridge piling on a lake in any way affects the progress of any other vessel.