They are able to exist because of people who are willing to work to support them. Even the communes rely on fertilizer, fuels, metals, foodstuffs from 2000+ years of increasingly efficient capitalists economies.
Don't get me wrong - I love spending a week to ten days out on the playa, and have never had such a wonder experience of "community" anywhere else - but I never, ever kid myself into believing that such a lifestyle is sustainable.
I don't like to use the word, "parasite", but I can't really think of a better word to describe the Haight street pan-handlers/garbage divers. They can't survive without people providing for them.