How could they possibly enforce that?
Just like they enforce everything else. Most of the people are law abiding and won't break the rules. There have always been closed areas near open areas, for fishing and hunting. As an analogy, what keeps someone from shooting a buck during the late Chiwawa or Entiat season without a permit and saying they shot it in Swakane general season? I fish those areas, and honestly don't think it's a huge problem. Bottom line, most of the people are law abiding, care about the resource, and won't break the rules.
BTW, while you don't see WDFW out all the time, I've heard of plenty of stories about them with optics watching for just that sort of thing.