The Wikipedia article addresses some of those issues. Feet tend to become preserved (specifically, it says “human feet have a tendency to become adipocere (a soap-like substance formed from body fat)”). The fact that some rotted off or were chewed off (rather than being cut off) means that this isn’t a serial killer’s calling card; if a killer is related, he would be just dumping bodies and the feet are an accident of decomposition.
Going back to “Why feet?” Unlike other body parts, feet tend to be in shoes and the shoes both float and make it harder for animals to eat the feet.
Given what’s in the Wikipedia article, I’m inclined to go with the 2004 Tsunami victim theory, though it sounds like the feet may be from multiple, unrelated events.
It’s not really that shoes with feet still in them are washing up - it’s shoes with feet still in them missing the body they were previously attached to.
A while back, in an article about cargo containers that fall off ships in high seas, and the resulting cargo that washes ashore from them, a strange fact turned up. When loads of athletic shoes were thus freed to float away, it was mostly right shoes that washed ashore. The article promised a scientific reason for the dexterous sneakers’ seaworthiness, but alas, I skipped the rest of the piece.
If you want to smack me for leading you on, you’ll have to find me first.
UPDATE: Genealogical DNA testing has identified one of the feet found in 2008 (in fact, it was the very Foot #6 mentioned in the OP) as belonging to a 17-year-old from British Columbia who disappeared in 2007 and was last seen leaving his house on bicycle at about midnight.
Looking at a map, I suppose it’s plausible that he jumped into the Fraser river to end his life, and that the currents subsequently carried his shoe-clad foot into the Salish Sea and down to the American side where it washed up.
Tragic case, but at least now his family has closure.
If interested in an implausible, conspiracy-theory enabling but generally entertaining mystery novel about lots of severed human feet in identical shoes washing up on a tropical beach, check out Crooked River, which probably was inspired by the Washington state feet cases.
