My understanding of the business model was similar to an organization
like the Better Business Bureau; the customers are the ones paying to
be certified, because being certified gives them some extra
legitimacy. BBB is able to do this because they have built up public
trust; essentially they're a reseller of public trust. If they do a
poor job of screening, it reflects poorly on their customers, and
trust in them is reduced.
CAs serve a similar function. If they have no public trust, what do
they have to sell? Surely people don't pay them 50-100 bucks for the
5 seconds of CPU time it takes to sign the certificate...