I basically agree with the conclusion you are reaching, but there is a perfectly good reason Zogby's approval ratings trend lower for Bush: As he does with ALL approval ratings tests, he offers people the option of choosing "fair." He counts "fair" as a negative rating, even though roughly half of people who vote "fair" would give thumbs-up in a thumbs-up or thumbs-down question. But he has always done this, for Clinton, Gore, Kerry, Bush, Dole, whoever; it is not indicative of bias.
if it is a self-selected sample, the poll has no statistical validity. it is the samle that is biased (i.e., non-random).