I agree with the conclusion. Posterous could fix this problem by implementing something like The Zucchini Method ( http://www.jgc.org/antispam/03152005-2150120647b00f4af9d3443... [PDF]). Basically, they could accept posts via email as long as the user included some hard to guess word (or other token) in the subject line.
Certainly not ideal. Typos would confuse matters and the idea of secret word authentication is not exactly common/obvious for the masses.