Good thoughts all. It sounds like most of you think ease of enforcement is the main reason.
To be the devil's advocate though, I'm kinda thinking that setting the season the same as seasonal lakes or streams to coordinate enforcement was an afterthought, rather than a primary reason since even WDFW can't point to a reason why the beaver pond restriction was originally put into place.
Here's an example of how arbitrary the season is: some lakes and streams are open all year while other are not. Why should ALL beaver ponds be aribitrarily closed? If enforcement was really a consideration, it'd be a hell of a lot easier to simply close ALL lakes and streams.
And what about lakes or ponds where there is no flowing inlet or outlet but yet where beavers have decided to set up house? I can think of several lakes that have beaver lodges on them but no dams since the lakes are spring-fed and their 'outlet' is simply a boggy wetland that seeps away slightly downhill. They're named lakes, not dammed up creeks, but closed nonetheless, because beavers happen to live there.
K