Why no arms race in this theory though? If spam is sure to ratchet up to meet whatever defense craigslist has today in order to continue to be effective, they might as well do a better job today as it is to not make themselves stand out so explicitly so they can get a higher click through rate. Seems to me there is some underlying reason or incentive why the current arms race on the half of the spammers might be stalled. Maybe there are enough people who fall for these scams as low effort as they seem to day, but if that were the case then craigslist is being a bit unethical allowing this susceptible population to be routinely victimized to ensure the rest of us don't get fooled by tougher spam.