Hey Fu,
I live in the Santa Monica 2 miles from the pier and beach. Well I went down to the beach to detect late one day a few weeks back, and I ran into a couple guys coming off the beach with detectors. I tried to talk with them but they didn't come across as really wanting to be sociable. They were not interested in anything I had to say except when I mentioned places in the area I like to go and then they were all ears. I quickly noticed they offered no return information so I shut up and offered little more myself. Once I stopped dishing out info they ignored me and kept packing to leave. The only thing I could really get out of them was they were down here from Ventura, but they could learn a thing or two about the golden rule. I could tell by looking at their gear they were setup for fast beach combing and little else.
So no I would say you are not alone and you are right, beaches are not self cleaning. Yes most cities run cleaning equipment over the sand, but it is not THAT good. I rarely find trash on our beaches which means they are being hunted well, not to mention it is really not the season for beach goers just yet.
The Shark