I believe they close the lower river to protect the kings that are starting to come up. Last year when I was looking for steelhead in the upper river above the H-18 Bridge, I saw several spawning kings. The Green is so over fished that they want to keep everyone from snagging and harassing the Kings. They are even employing new special bobber only rules this year near the river mouth so when the Silvers come in, the snaggers in Seattle will stop chucking monster treble hook's at them. I live near the Green (in Covington) and I am always disgusted on what I see going on there. Last year on the upper green near the Tacoma headworks, there is a railroad bridge. I was in the canyon below and a husband and wife were on the other side. The Husband was fishing and the wife was just sitting there. Some @#$% kids decided to pee from the bridge on the lady. Their fishing trip was ruined and so was mine. I would rather drive for two hours and fish where there are fewer people and have a quality fishing experience (even if I don't catch anything) than put up with what I see on the Green. I will say this though. On October 16 and for a few weeks after, you will still find me at my favorite hole in the river attempting to get the fussy Silvers to test my 8 wt.
If you want to see some combat fishing, go the Meat Hole (where Soos Creek come in) and check out the Zoo on October 16th. Even before the 16th you will see lots of people breaking the rules and fishing above the 277th Bridge.